Giants causeway IircWhinstone is basalt, or a volcanic magma in this case that didn't make it to the surface. See http://en.wikipedia.org/wiki/Whin_Sill
It's in a flat-lying 'seam' (aka a sill) under much of N England and the Borders, and it's also the crag that Hadrian's Wall is built on.
Basalt is the stuff that is erupted in Iceland right now, and also forms much of the Hebrides - think Staffa/Fingal's Cave?
Anything else you'd like to know?