Posted: Nov 05, 2018 7:57 am
by Adco
Cito di Pense wrote:
It's painful to see you keep puzzling about this three years on with nothing better to show than a few crude photographs. You haven't been able to identify whether this is an igneous, sedimentary, or metamorphic texture. You haven't reassured me that the white inclusions are quartz. although the fracturing is suggestive. For all we know, this could be a metamorphosed sedimentary rock that had irregularly-shaped quartz pebbles in it. You can't even show which way is "up" in the rock as it was originally deposited. If it is a metamorphic texture, good luck with finding "up" if all you have is that. You don't know which way was up when the differential weathering was taking place that removed some of the inclusions. There are no questions about this rock that you've been able to answer definitively. How dense is the matrix, which you say is 'blue". If it's quite dense, odds-on this is a metamorphic rock. It's possible that the clasts or pebbles that are missing did not dissolve, but were simply washed away when the matrix around them weathered. See what I mean?

If the pebbles are quartz, it's unlikely they dissoved. Quartz is pretty resistant to weathering in a water-saturated environment, but it's not even clear from the photos that these are crystals of quartz and not chunks of polycrystalline quartz or some other light-colored mineral.
I see exactly what you mean and I'm sorry it pains you that I haven't been able to show you anything of worth. That's exactly the reason I started this thread. I don't know. I'm not a geologist. I was merely looking at these formations in my garden and wanted to discus and hopefully get some answers from more learned people. I tried looking on the net but there is too much to learn casually. I see from your answer that you know more than me about rocks and things but still not enough to give a proper answer either. I posted the second set of pictures after a discussion with halucigenia who knows what he is talking about. Very helpful I must say.