I'm wondering if it's possible to better identify some porphyritic rocks that I'm finding here in Baltimore county, eastern-most part of the Piedmont Plateau. It's just west of Baltimore City limits. I don't know a good source that could help. This rock very well might end up my 7yo son's upcoming school project, so any assistance in better identifying/describing it would be much appreciated!
The first picture below shows the porphyry in question. It's the rock on the left side of the picture. The items on the right of the picture are some small volcanic glass samples found in the same area during the same dig. I pulled these out of a creek bank, about 6 to 12 inches to down. The porphyry is a couple of inches in length. The region that these samples came from contains quite a lot of quartzite, mica, and iron bearing minerals such as goethite. There are smaller amounts of copper bearing minerals as well. The volcanic glass appears to have some iron mineralization in that I can pick them up with a magnet. Couldn't pick-up or move porphyritic rock with the same rare-earth magnet, so it might not contain ferrous minerals.
The next two pictures are close-ups of the porphyritic rock showing some of the crystallization contained within. Apologies for any difficulties in seeing the crystals. Many are brownish in color and blend in quite well with the grey body of the porphyritic rock. There are some clear crystals in the rock as well. The crystals are quite small, so its tough to carry out any meaningful analysis on them.
Regards,
Mark L.If you're in the Baltimore area, I don't think you'll find a porphyry, which is an igneous rock. You will find metamorphic rocks; the larger grains you're seeing would then be called porphyroblasts. As to the underlying rock type, I'm not sure, but maybe a gneiss or schist. 
I would suggest that you contact the geology department of a local university and talk to one of the geology professors. Doug,
Appreciate the reply, but apologies for having to disagree with your comments. I always worry about doing so in such forums, since I'm often the least experienced rock hunter in the discussion. Sometimes one must dare to be wrong. So, here it goes...:)
Baltimore county certainly has igneous rocks. There's gabbro all over the place. The Baltimore Gabbro Complex sits under a large swath of western Baltimore county with out croppings in Ellicott City. Moreover, I've found several samples of volcanic glass such as obsidian (not in earlier picture) and what appears to be cooled lava ( shown in the picture -- not best view, admittedly) in the area.
Perhaps, you meant that metamorphic rocks are far more common than igneous rocks in this area, which is probably true. Honestly, I didn't expect to find any extrusive igneous samples here. As I mentioned, gabbro is very common, but this is an intrusive (plutonic) igneous rock. However, hard to argue with the rock is sitting in my hand. Admittedly, haven't found much. Hence, it might have found its way here from elsewhere at some point in the distant past.
As for the rock in question, certainly possible that it's not porphyry. However, it's nowhere close to being gneiss or schist. The fine grained portion that makes up the body of the rock is not layered. It's one solid chunk and doesn't look like any metamorphic rocks that I've seen. Could be that it some other type of conglomerate.
The area that these rocks come from sits just past one of the edges of the Baltimore Gabbro Complex, according to Maryland Geological Survey (MGS) Maps. This is also the eastern most region of the Piedmont Plateau.
Regards,
Mark L. Well, I can always be corrected, even tho I'm a geologist by training I'm not GeoGuru the Great. Like Rock suggested, take it to a local college's geo department - there certainly have to be some in that area. Anywho, let us know what it is....we also like to learn new things. 
I'm with Doug and Rock, take them to a local Geology Dept. to have a look-see.
Finding volcanic glass, i.e. Obsidian, anywhere on the east coast USA, seem pretty unlikely too.
MRH Hi Mark, I'm a retired geology professor who grew up and cut my "rockhound" teeth in Maryland (College Park) so I do know the area. Making identifications from photos is notoriously tricky - but, here goes. I think your "volcanic glass" is man-made slag. There is plenty of that around the Baltimore area, as there were plenty of old iron workings there, some dating back to pre-Revolutionary War time. The "porphyry" looks to be to instead be a breccia - a rock made up of pieces of other rocks. A lot of the fragments look like angular pieces of quartz - a few look blue, and blue quartz is not uncommon in the Piedmont area of Maryland. from the matrix - the material between the fragments - I'd say the rock was metamorphosed, possibly a metamorphosed sedimentary rock or event a meta-volcanic rock. Much of the bedrock in the Baltimore area is metamorphic rock from the various older mountain-building episodes, all of which have fancy names. I've been working through the "Maryland" page on MINDAT http://www.mindat.org/loc-14027.html and you will find a lot of interesting stuff there, including leads on reference books on Maryland geology for the general public. You'll also find a link to Maryland's state geolgoical survey. Maryland has a very diverse geology, and its was fun to look for rocks there when I was growing up. I wish your son good luck on his science project. 
I agree. These look like slag that entrained some pebbles as it cooled.
I don't see anything that appears to be a "natural" porphyritic rock. All
Can't tell you how much I appreciate your patience and input. This info will help greatly. Thanks for putting me on the right trajectory! Mark, You can't be far from College Park, Univ of Md has a good geology department. I studied there. I think Bill Cordua is right-on, but you still might stop by the geo dept to get their take on your rock and to establish contacts. I'm about 45 mins from UM College Park, so I am pretty close. I'll check out their dept, but will probably take some time to collect a larger sample size first. I've been quite surprised by the high variability in rock and mineral characteristics over small distances in the area I've been searching, so I'm sure additional exploration will result in more interesting questions. WRT the original question, I agree that Bill Cordua is almost certainly correct. Breccia seems much more plausible than my guess. :)
On the vertical plane, the transition between clay types and eventually sand (a few feet down) is fascinating. Starts with black soil/ clay mix with lots of organic material at the top. This is followed by a fine red clay, gray in some places. This is very much like pottery clay. Most areas have a dense blackish/dark blue band of mineralized clay running through it. Perhaps some variety of iron. Next is a thin layer of porcelain white clay, with a tinge of pink in some areas. The white clay seems to have been compressed into layered slate-like rocks in some spots. Under the clay is coarse beach-like sand. Not sure how deep the sand is. Rock types vary with depth somewhat with dark colored quartzite and chunks of gabbro becoming more common with depth. Lots of milky quartz at all depths.
Rock and mineral variation seems most pronounced on the horizontal plane. Right now, I'm attempting to follow a gradient of increasingly mineralized quartzite and other rock. There seems to be a discrete band of quartzite containing mica flakes and what appears to be pyrite. When polished, these rocks are quite beautiful.
