[QUOTE 3428317, member: 9609"]In a recent thread there was a discussion about geology and different types of stone which I found particularily interesting. And as I am about to do some work with local stone (a dry stone wall in the garden) I am quite curious as to know more about it. It would generally be known around these parts as Whinstone, but can anyone help me be more specific.. It is basically field stone that the farmer has dumped in one corner.
What I know so far; according to this web site
Link the geology of the area is
this is it in the corner of the field
This is a piece cracked apart with a hammer (sometimes a slight smell of flint when broken)
And this is a piece cut with a diamond tip blade
and I will give
@McWobble a shout as he seems to know about geology things[/QUOTE]
I'd really need to see the rock up close and personal, with my hand lens (and ideally have a thin section of it and a petrological microscope - but you probably don't have those!).
However... The rock in question is fine-medium grained and dark grey with small interlocking crystals visible. That strongly suggests that it is igneous and intermediate in composition (more silica than basalt, but less than granite). The size of the crystals implies that the rock solidified from its parent melt quickly and close to the surface. The colour of the weathered rock is rust, most likely from oxidised iron - again, this suggests an intermediate or mafic (basaltic) composition.
The dark flecks look like biotite mica, the white ones are probably feldspar. I can't see any olivine crystals, which would unambiguously identify it as a basalt, nor can I see any glassy silica crystals which would mean it's an andesite. My first instinct was it's an andesite, but the weathered rusty appearance suggests it's a basalt. On the other hand, the cut section is far too pale - that hints that it's andesite! Any chance of a close up photo of the cracked section - that might help. Have a look and see if you can spot any glassy and translucent silica crystals, or olivine - it'll appear as a dark green crystal.
What size are the crystals? That also has a bearing: basalt and andesite are both fine grained with crystals smaller than 0.25 mm. If they're bigger then it'll either be the medium grained basaltic equivalent dolerite or micro-diorite.