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The Minerals of Castle Point, Hoboken, NJ (Darton 1882)

Last Updated: 6th May 2008

By Daniel Russell

N. H. Darton On The Minerals of Castle Point,
Hoboken, Hudson County, New Jersey (1882)

In 1883, Nelson Horatio Darton, an 18 year man old from Brooklyn, New York, published a lengthy piece in the Scientific American Supplement about the minerals of several of the more interesting mineral sites around New York City. Born in 1865, Darton apprenticed in his grandfather's pharmacy at the age of 13 and developed a sound understanding of practical chemistry. In 1886 he would join the United States Geological Survey, where he remained until his death in 1948.

The “Castle Hill” locality of which Darton writes is better known today as “Castle Point” in Hoboken, Hudson County, New Jersey. The site is a classic serpentine outcrop, forming the highest promenance in Hoboken. It commanded a stunning view of the Hudson River. Castle Point comprised the southern end of “Elysian Fields,” a large open area used by the population of early 19th Century Manhattanites as a park; it would later be incorporated into the property of Stevens Institute.

Castle Point was the type locality for both the minerals Brucite (named for early US mineralogist Archibald Bruce) and Hydromagnesite (named for its composition). Darton notes the presence of “Chromic Iron” (chromite) in his list of species, which is a fairly common accessory mineral in serpentinized ultramafic rocks; it does not appear on the Mindat species list, and any documented specimens from the site should be examined for its presence.

Darton, Nelson Horatio
The Mineralogical Localities In And Around New York City, And The Minerals Occurring Therein

Scientific American Supplement Vol. XIV No. 344
NEW YORK, August 5, 1882

Nelson Horatio Darton
Hoboken.--The locality represented here is where the same serpentine that we met on Staten Island crops out, and is known as Castle Hill. It is a prominent object in view when on the Hudson River, lying on Castle Point just above the Stevens Institute and about a mile north of the ferry from Barclay or Christopher Street, New York city. Upon it is the Stevens estate, etc., which is ordinarily inaccessible, but below this and along the river walk, commencing at Fifth Street and to Twelfth, there is an almost uninterrupted outcrop from two to thirty feet in thickness and plentifully interspersed with the veins of the minerals of the locality, which are very similar to those of Staten Island; the serpentine, however, presenting quite a different appearance, being of a denser and more homogeneous structure and color, and not so brittle or light colored as that of Staten Island, but of a pure green color. The veins of minerals are about a half an inch to — in the case of druses of magnesite, which penetrate the rock in all proportions and directions — even six inches in thickness. They lie generally in a perpendicular position, but are frequently bent and contorted in every direction. They are the more abundant where the rock is soft, as veins, but included minerals are more plentiful in the harder rock. There is hardly any one point on the outcrop that may be said to be favored in abundance, but the veins of the brucites, dolomite, and magnesites are scattered at regular and short intervals, except perhaps the last, which is most plentiful at the north end of the walk.

Magnesite — This mineral, of which we obtained some fine specimens on Staten Island, occurs extremely plentifully here, constituting five or six per cent. of a large proportion of the rock, and in every imaginable condition, from a smooth, even, dark colored mass apparently devoid of crystalline form, to druses of very small but beautiful crystals, which are obtained by selecting a vein with an opening say from a quarter to a half-inch between it and one or, if possible, both points of its contact with the inclosing rock, and cutting away the massive magnesite and rock around it, when fine druses and masses or geodes may be generally found and carefully cut out. The crystals are generally less than a quarter of an inch long, and the selection of a cabinet specimen should be based more upon their form of aggregation that the size of the crystals. Nearly all the veins hold more or less of these masses through their total extent, but many have been removed, and consequently a careful search over the veins for the above indications, of which there are still plenty undeveloped or but partly so, would well repay an hour or more of cutting into, by the specimens obtained. Patience is an excellent and very necessary virtue in searching for pockets of minerals, and is even more necessary here among the multitudinous barren veins. One hint I might add, which is of final importance, and the ignorance of which has so far preserved this old locality from exhaustion, is that every specimen of this kind in the serpentine, of any great uniqueness, is to be found within five feet from the upper or surface end of the vein, which in this locality is inaccessible in the more favored parts without a ladder or similar arrangement upon which one may work to reach them. Here the veins will be found to be very far disintegrated and cavernous, thus possessing the requisite conditions of occurrence (this is also true of Staten Island, but there more or less inaccessible) for this mineral and similar ones that occur in geodes or drused incrustations, while it is just vice versa for those occurring in closely packed veins, as brucite, soapstone, asbestos, etc., where they occur in finer specimens, where they are the more compact, which is deep underground. This is also partly true of the zeolites and granular limestone species with included minerals. I do not think there is any rule, at least I have not observed it in an extended mineralogical experience; but if they favor any part, it is undoubtedly the top, as in the granular limestone and granite; however, they generally fall subordinate to the first principle, as they more frequently, in this formation, with the exception of chromic iron, occur not in the serpentine but in the veins therein contained; for instance, crystals of dolomite are found deeper in the rock as they occur in the denser soapstone, which becomes so at a more or less considerable depth, with spinel, zircon, etc., of the granular limestone. They occur generally in pockets within five feet from the surface, but they can hardly be called included minerals, as they are rather, as their mention suggests, pockets, and adjacent or in contact with the intruded granite or metamorphosed rock joining the formation at this point. This is seemingly at variance when we consider datholite [datolite], but when we do find it in pockets a hundred and fifty feet below the surface, in the Weehawken tunnel, it is not in the trap, but on the surface of what was a cleft or empty vein, since filled up with chlorite extending from the surface down, while natrolite, etc., by the trap having clefts of such variable and often great depth, allowed the solution of the portion thus contributed that infiltered from the surface easy access to the beds in which they lie, the mode of access being since filled with densely packed calcite, which was present in over-abundance. This is not applicable to serpentine, as the clefts are never of any great depth, and the five feet before mentioned are a proportionately great depth from the surface. As I mentioned in commencing this paper (Part I), every part of the success of a trip lies in knowing where to find the minerals sought; and by close observation of these relations much more direction may be obtained than by my trying to describe the exact point in a locality where I have obtained them or seen them. There is much more satisfaction in finding rich pockets independently of direction, and by close observance of indications rather than chance, or by having them pointed out; for the one that reads this, and goes ahead of you to the spot, and either destroys the remainder by promiscuous cuttings, or carries them off in bulk, as there are many who go to a locality, and what they cannot carry off they destroy, give you a disappointment in finding nothing; consequently, I have considered that this digression from our subject in detail was pardonable, that one may be independent of the stated parts of the locality, and not too confidently rely on them, as I am sometimes disappointed myself in localities and pockets that I discover in spare time by finding that some one has been there between times, and carried off the remainder. The characteristics of magnesite I have detailed under that head under Pavilion Hill, Staten Island; but it may be well to repeat them briefly here. Form as above described, from a white to darker dirty color. Specific gravity, 2.8-3; hardness, about 3.5. Before the blowpipe it is infusible, and not reduced to quicklime, which distinguishes it from dolomite, which it frequently resembles in the latter's massive form, common here in veins. It dissolves in acid readily with but little effervescence, which little, however, distinguishes it from brucite, which it sometimes resembles and which has a much lower-specific gravity when pure.

Dolomite — This mineral has been very common in this locality. It differs, perhaps, as I have before explained, from magnesite in containing lime besides magnesia, and from calc spar by the vice versa. Much of the magnesite in this serpentine contains more or less lime, and is consequently in places almost pure dolomite, although crystals are seldom to be found in this outcrop, it all occurring as veins about a half-inch thick and resembling somewhat the gurhofite of Staten Island, only that it is softer and less homogeneous in appearance. Its color is slightly tinged green, and specimens of it are not peculiarly unique, but perhaps worth removing. Its characteristics are: first, its burning to quicklime before the blowpipe, distinguishing it from pure magnesite; second, its slow effervescence in acids. Besides these, its specific gravity is 2.8, hardness, 8.5; from calcspar it cannot be distinguished except by chemical analysis, as the two species blend almost completely with every intermediate stage of composition into either calc spar, or, what occurs in this locality, aragonite, similar in composition to it, or dolomite. The color of the last, however, is generally darker, and it cleaves less readily into its crystalline form, which is similar to calc spar, and of which it is harder, 3.5 to 3 of calc spar.

Aragonite — This mineral, identical in composition with calc spar, but whose crystalline form is entirely different, occurs in this locality in veins hardly recognizable from the magnesite or dolomite, and running into dolomite. It is not abundant, and the veins are limited in extent; the only distinguishment it has from the dolomite, practically, is its fibrous structure, the fibers being brittle and very coarse. If examined with a powerful glass, they will be seen to be made up of modified long prisms. The specific gravity is over 2.9, hardness about 4, unless much weathered, when it becomes apparently less. There are some small veins at the north end of the walk, and in them excellent forms may be found by cutting into the veins.

Brucite — This mineral occurs here in fair abundance, it being one of the principal localities for it in the United States, and where formerly extremely unique specimens were to be obtained. It has been pretty well exhausted, however, and the fine specimens are only to be obtained by digging into the veins of it in the rock, which are quite abundant on the south end of the walk, and, as I before noted, as deep as possible from the top of the veins, as it is a closely packed mineral not occurring in geodes, druses, etc. Two forms of it occur; the one, nemalite, is in fibers of a white to brown color resembling asbestos, but the fibers are brittle, and hardly as fine as a typical asbestos. It is packed in masses resembling the brucite, from which it only differs in breaking into fibers instead of plates, as I have explained in my description of that species (see Part II). They are both readily soluble in acids, with effervescence, and infusible but crumble to powder before the blowpipe, or at least become brittle; when rubbed in mass with a piece of iron, they phosphoresce with a yellow light; specific gravity, 2.4, hardness, 1.5 to 2. Its ready solubility in acids without effervescence at once distinguishes it from any mineral that it may resemble. The specimens of nemalite may be more readily obtained than the brucite but fine specimens of both may be obtained after finding a vein of it, by cutting away the rock, which is not hard to do, as it is in layers and masses packed together, and which maybe wedged out in large masses at a time with the cold chisel and hammer, perhaps at the rate of three or four cubic feet an hour for the first hour, and in rapidly decreasing rate as progress is made toward the unweathered rock and untouched brucite, etc.

Serpentine — Fair specimens of this may be obtained of a dark oil green color, but not translucent or peculiarly perfect forms. The variety known as marmolite, which splits into thin leaves, is plentiful and often well worth removing.

Chromic Iron[Chromite?] — Crystals of this are included in the denser rock in great abundance; they are very small, seldom over a few lines in diameter, of an iron black color, of a regular octahedral form; sometimes large crystals may be found in place or in the disintegrated loose rock. I have seen them a half inch in diameter, and a half dozen in a small mass, thus forming an excellent cabinet specimen. By finding out by observation where they are the thickest in the rock, and cutting in at this point, more or less fine crystals may be obtained. This is readily found where they are so very abundant, near the equidistant points of the walk, that no difficulty should be encountered in so doing. These characteristics are interesting, and if large specimens cannot be obtained, any quantity of the small crystals may be split out, and, as a group, used for a representative at least. Before the blowpipe it is infusible, but if powdered, it slowly dissolves in the molten borax bead and yields a beautiful green globule. The specific gravity, which is generally unattainable, is about 4.5, and hardness 5 to 6. Its powder or small fragments are attracted by the magnet. A few small veins of this mineral are also to be found horizontally in the rock, and small masses may be obtained. They are very rare, however. I have seen numerous agates from this locality, but have not found them there myself. They may be looked for in the loose earth over the outcrop, or along the wall of the river.


The 1900 Annual Report of the Geological Survey of New Jersey contains a notice by Albert H. Chester of the acquisition by the Survey of a specialized collection of Castle Point minerals, purchased from none other than Tiffany gemologist George F. Kunz. He offered analyses of two materials from the site… “Deweylite” (a mixture of lizardite and stevensite) and “Mesitite” (later shown to by an impure magnesite):


During the past year a large collection of minerals from Hoboken, recently purchased from Mr. George F. Kunz, has been thoroughly examined and labeled. Though not a great variety is found, the collection well represents all the minerals that have been described from that locality, and from among them we are now able to add two species not heretofore noticed there, nor, indeed, at any other locality in the State. This collection comes from the well-known "Castle Point," which is now completely covered with buildings and other improvements, so that it is doubtful whether any more specimens will ever be obtained there. Among them are many specimens, some of them large and fine, of the mineral hydromagnesite, which is not at all common anywhere. There is also a large number of specimens of nemalite, the fibrous variety of brucite, a mineral almost unknown elsewhere.


Among these specimens some were noticed as incrustations on the serpentine, having all the characteristics of deweylite, a mineral not heretofore identified from this State, but to be expected in such an association of minerals. Analysis shows it to have the following composition:

Analysis of Deweylite

SiO2 38.20 per cent
Fe2O3 0.94 "
Al2O3 0.80 "
CaO 4.76 "
MgO 31.32 "
SO3 0.11 "
Cl 0.15 "
CO2 3.70 "
H2O 100° 7.67 "
H2O red heat 12.09 "
Total 99.74 "

This mineral is seen as an incrustation, about one-half an inch in thickness, on the serpentine, probably originally filling a seam; is of a yellowish brown color, looking like gum arabic, and is very brittle and crumbly. It has the physical characteristics of deweylite, and its chemical composition, as seen from the above analysis, the specimen analysed showing a small admixture of aragonite, the carbon dioxide and lime almost exactly neutralizing each other, and some other slight impurities. It is proper, therefore, to add this to the list of minerals already found in New Jersey.


Another seam in the serpentine is filled with what appears to be carbonate of iron and magnesium, mixed with some other minerals. As it occurs in a seam only about one-fourth of an inch in width, and the other minerals cannot easily be separated from it, it was analyzed as a whole, and an attempt was made to get at the proportions of the various minerals by calculation. The results of this analysis are:

Analysis of Mesitite

SiO2 21.90 per cent
Fe2O3 6.97 "
CaO 1.52 "
MgO 37.33 "
CO2 7.10 "
H2O 100° 6.41 "
H2O red heat 18.56 "
Total 99.79 "

The lime was first calculated with enough carbon dioxide to make aragonite. The silica then took out its equivalents of magnesia and water to form deweylite, the remaining carbon dioxide was calculated with ferrous oxide and magnesia to mesitite, and the remainder of ferrous oxide, magnesia and water, of which latter there is an excess, belongs to the nemalite, which is evidently present. The appearance and blow-pipe characters of part of this material point so clearly to mesitite that I feel justified in accepting the results of this analysis and calculation, which show the filling of the seam, deducting the aragonite, to consist of about 55 per cent, of deweylite, 30 per cent, of nemalite and 15 per cent, of mesitite, thus adding another to our list of New Jersey minerals.

Chester, Albert H.
Part IV — Mineralogical Notes And Explorations
Annual Report Of The State Geologist For The Year 1900
Trenton, 1901

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