Sure, agreed. Timescale is obviously important, but we can accelerate or decelerate that as we wish. Again, from a purely geological point of view, feldspars can deterioate into in situ clays. That would weaken the surface formation but give you finest Whitewall china! Er... nevermind.
> My personal favourite for the rock still is quartzite...
But it's *boring*... only kidding. On my mapping excursion to Islay back in '98, a good 40% of the outcrop was monotonous quartzite. Saved time though, and made for a very pure water to trickle down into the peat bogs north of Laphroaig, Lagavulin and Ardbeg.
> A pegmatite with a whitish feldspar plateau above a hydrothermal
> quartzite enclosure... which was made to blossom up through the
I'm glad *you* mentioned that Joerg... ;o)
Now the terminology threshold has been broken, I'll weigh in and suggest some additional elements which might help Joerg's example, and give us the best of all worlds.
We all like using examples of RW rocks, as we know what they look like, and, after all, they are kind of cool. Sadly geologists actually stop looking at rocks after the first few weeks as an undergrad. It's all a big deception. The problem is process... which can be queried, and most of us won't necessarily understand.
If we assume rocks to be gods' bodies and bones, I think Joerg's suggestion can work really well. Imagine the plateau on which Whitewall stands to be the end of a giant bone, thrust slightly out of the ground. Now, the endless procession of myth and time has weathered this down to plateau-like form.
The top of the plateau is therefore a cross-section across the mighty bone. The hard outer edges of this bone can be a nice white rock, strong if that is what people wish. Joerg's favourite quartzite would fit the bill, or a granite composed mainly of quartz and feldpsars [or a syenite... sorry ;o)], or a prominently feldspathic bit of pegmatite*.
*folks, pegmatite means a rock with very big crystals.
This could merge from one into the other--just as pegmatites are frequently layered, with quartz and feldspar strong areas. Now, the inside, well, that's the softer marrow! A perfect suggestion for this is a limestone. This works for all your tunnels, caves and wells, and acts as an aquifer, as I've alluded to elsewhere. On top of all that, the relatively impermeable outer rock keeps the water in.
So we have all the rocks people like, a mix of strength, aesthetic appeal and cool tunnels... compounded by a very simple mythological basis: Jajagappa buried his bone here and forgot about it. Or... er... something similar.
All the best,
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