Page:A Treatise on Geology, volume 1.djvu/150

 134 the best parts of the Grampians; while the valleys, sometimes richly wooded and watered (Festiniog, Dolgelly,) and sometimes dreary and solitary (Llanberis, Beddgelert), furnish, even without their wild lakes and rough cascades, every possible variety of pictorial accompaniment.

If we consider how all these circumstances depend upon the general conditions of a forcible elevation of rocks of different qualities into an atmosphere competent to produce upon them unequal chemical effects, and on their disintegrated particles to nourish correlative vital phenomena, we shall see how trifling is the enjoyment of beautiful nature which they experience who are satisfied to gaze on effects with the painter, and seek not their appointed causes with the geologist.

Igneous Rocks are associated with the argillaceous slates in every district where they appear in the British islands. Granites touch the slates in Cumberland, in Cavan and Arran; porphyry and greenstone are abundant in the Cumbrian mountains and Snowdonia, both in dykes and partially stratified masses. Mineral veins are found nowhere so abundantly as near the granites, or other igneous rocks; a circumstance which combines with many other facts to demonstrate the dependence of mineral veins on some peculiar agency of subterranean heat. Quartz, the most frequent matrix or vein stuff of these mineral veins, is very often found ramifying into the fissures and cracks of the slate rocks, without any metallic admixtures.

Judging from personal observation, as well as recorded phenomena, we should say the effects of locally developed igneous agency are much more frequent among the rocks of the slate system, than in those of earlier date, and that dykes, veins, and interspersed beds of porphyry and greenstone, are more abundant and varied. The circumstances observed in Cumbria and North Wales of the porphyry beds being subject to the same flexures and inclinations as the slates, leads to the inference that they were effused, as lava sometimes is, on