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Beach Rocks (Plutonic?)

Posted by P. Michael Hutchins  
Beach Rocks (Plutonic?)
February 26, 2012 08:40PM
(I couldn't figure out what forum to post this in.)

I collect mineralogically interesting beach rocks from a geologically-complex area around Boston (Marblehead). Some look to me like they're displaying flow banding; some, magmatic immiscibility. Some have ~micro-graphic granite~.
(the graphic texture is (almost) too small to see w/ the naked eye)

Some have almost agate-like bands w/ xls on one side, all swirling around (folded?).

And so on.

I haven't figured out even where to look for information about these / this kind of rocks.

My first question was:

What minerals would this ~magma~ have turned into if it had cooled slowly enough?

Now I just want to learn enough to be able to look at one of these rocks and read its story, tell how it formed.

(I'm not a geologist; just love geology, mineralogy, etc.)
(MIT: Pure Math, Computer Science)

I'd treasure advice on where all to look, what discipline(s) this might fit into, technical terms that would help in searches, etc.
avatar Re: Beach Rocks (Plutonic?)
February 26, 2012 10:23PM
Some photos would help. What you have might not even be igneous which is why photos of the specimens are needed. As far as information, just type in igneous rocks or something to that effect into Google, Yahoo, etc... Better yet, find a recent copy of a college petrology textbook and it will give you more information than you ever wanted to know.

Edited 1 time(s). Last edit at 02/26/2012 11:11PM by Paul Brandes.
avatar Re: Beach Rocks (Plutonic?)
February 28, 2012 12:02AM
I don't know if this can be of help in any way, but it looks interesting:


Re: Beach Rocks (Plutonic?)
February 28, 2012 08:58PM
Thanks, guys!

I was definitely remiss in not including any photos!

Here's one of each rock that I have out for this purpose. I can certainly make zooms if they'd help.
open | download - P2280015.JPG (294 KB)
open | download - P2280016.JPG (193.4 KB)
open | download - P2280017.JPG (99.7 KB)
avatar Re: Beach Rocks (Plutonic?)
February 28, 2012 11:47PM

The first one looks like almandines in some kind of shist,
Third one is some variety of chalcedony, probably flint
Second... I am not quite sure. For sure it is sedimentary rock, I see a lot of calcite, I think it has grains, so maybe it is sandstone, or even limestone with grains.

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