SLATES OF WALES AND TH J LJK 



felsites, which would have yielded materials exactly similar to the 

 micaceous clay slates, for when the particles of mica are so exceed- 

 ingly fine, they are not separated from the kaolin. 



Cambrian Slates. Associated with the slates of Bethesda, near 

 Bangor, is a bed of felsitic ash. Some of the fragments are an im- 

 perfect pumice. There are quartz grains, which appear to have been 

 derived from a quartz felsite, for they occasionally enclose felsitic 

 material similar to the felsites of the neighbourhood. There are fre- 

 quently grains and needles of magnatite, and sometimes black grains 

 of basalt. Fragments of augite appear to have been altered into 

 chlorite. If, observes Dr. Sorby, the micaceous base of this ash 

 were worn down into dust in a volcanic crater, or more completely de- 

 composed by weathering, and the material afterwards sorted by gentle 

 currents, it would yield a deposit corresponding in all essential parti- 

 culars with the fine-grained slates of Penrhyn and Llanberis. Other 

 beds in the same group of rocks contain little mica and much kaolin, 

 and were evidently derived from an igneous rock which contained 

 relatively more felspar than that which yielded the Llanberis slates. 



When examined under the microscope, it is amazing to see how 

 frequently some beds, like the pencil slate of Shap, consist almost 

 entirely of mica. These Shap slates show the particles disturbed by 

 pressure, and in process of being arranged into the parallel films 

 which constitute cleavage. 



The Green Slates of Cumberland and Westmoreland are well 

 known to be almost entirely derived from volcanic ashes. Specimens 

 from Rydal and Langdale show the original material very little altered. 

 Sometimes the fragments are ^th of an inch in diameter, and are 

 derived from very vesicular lavas or rocks passing from the condition 

 of pumice through perfect glass to a devitrified felsite, owing to the 

 development in it of minute crystals. Quartz is frequently absent, 

 just as in the South Pacific red clay ; and when occasional grains of 

 quartz are found, the cavities in them are free from fluid. At Amble- 

 side large fragments of pumice occur with their cavities infiltrated 

 with calcite exactly as is the case at the present day in the fine-grained 

 mud of the ocean. Frequently, however, the rock is more changed, 

 so that it would be impossible to tell whether the ash was erupted 

 and changed in the water, or whether heat took any part in the pro- 

 cess. Much of the felspar, augite, and garnets may have originally 

 belonged to the ash, but other minerals may have been formed where 

 they are now met with. The Green Slates of the Lake district are 

 formed of fragments, so compressed in the plane of cleavage that it is 

 impossible to tell whether they were derived from felsite or recon- 

 structed out of an older slate. 



The slates of Loch Awe, although presenting an ordinary appear- 

 ance to the eye, show under the microscope all the characters of a 

 very fine-grained schist. The slates of Moffat are of the usual granular 

 type, mixed with others which are highly micaceous, in which are 

 3 or 4 per cent, of glassy fibres like the Pele's hair of the Sandwich 

 1 slands. 



