Maybe a Fossil-please help ID.

[mrsljwalker]

I found this in a tangle of tree roots along the American River. The water is low, so it's a great time to look for cool rocks. When I look at this I see either crystalized mineral deposits or tiny fossils, maybe plant or insect material. Please let me know if you have any ideas about it.

 

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[GatorBoy]

I can't help you with identification but that's definitely a neat one

 

[mrsljwalker]

Thanks, Gator. I will have to do some research and see what I come up with.

 

[Harry Pristis]

My guess is that you have phenocrysts in an igneous groundmass, but geology is not my thing.

 

[BurntBear]

I have found similar, yet different specimens in North Carolina. My shapes were caused by an algae-like plant that lived on the rock and iron oxides stained around the growth and created a similar pattern. Yours has little "branches" or "fingers" which is indicative of plant material. I won't rule out Phenocrysts, but I've never seen any look like that. Usually seen in granites with unusually large plagioclase feldspars. -Luke

 

[mrsljwalker]

I looked at quite a few phenocryst specimens to see if any were an exact match, but they all seem to be very chunky, and none seemed to show the feathery edges that are so plant-like. I like the idea of a rock marked by plant growth, but this rock is marked the same way 360 degrees around. I think usually lichens and similar plants tend to grow on one side, rather than uniformly all around the object. If it was not such a pretty rock, I would like to split it to see if the deposits are throughout the material. I may consider taking it to a local Geology department to see what they can tell me. Thank you for the good ideas, I am still working on finding the answer.

 

[Harry Pristis]

Phenocrysts don't come in standard shapes and sizes. It cannot be a plant fossils in igneous rock, assuming the crystals are distributed throughout the matrix. And, it would be a safe assumption that the surface you see has been ground down from a much bigger boulder by the American River. Therefore, we are looking at the interior, now exterior, of the matrix.

If you don't want to break this rock, or have it sawed in two and polished, go back where you found this one to look for another example that you can break.

 

[mrsljwalker]

Thank you, Harry. I will be heading back to the river in hopes of finding another sample. Most of the rocks in my area are remnants of mine tailings so they have likely been moved and mixed up prior to a 150 year journey in the river. I may try looking closer to the historic mining sites around the area as well.