Squeezed from the rock like serum from a wound, it congeals in the cracks. This must have been the source of those large lumps in the stone walls. Elsewhere, it fills in voids in stressed rock like some kind of mad spaghetti. Ultimately, though, quartz is tougher than slate, and survives as pebbles long after the country rock has been eroded away. I would be willing to wager that some of the rounded pebbles on inaccessible Pentargon beach far below me are made of quartz.
Cliffs are liable to cracking.
Take extra care. And it's true. A stack of shale is teetering outwards into the void. It is hard to escape a shiver of apprehension as you imagine the block tumbling over and over to smash to pieces far below. The next haven up the coast is called Crackington, the name encapsulating the precariousness induced by erosion.