MICA SLATE. 



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rous and large seams of quartz, running parallel with llie slaty struc° 

 ture of the rock, and increasing in size, as they approach the granite. 

 The quartz has a greasy^aspect, and is evidently, of cotemporaneous 

 formation with the mica-slate and granite. 



Mica-slate has a near affinity to clay slate ; and, as I have arranged 

 the latter with rocks of the second class, it may perhaps be doubted 

 whether mica-slate should not also have been transferred to the same 

 class. No well characterised rocks of mica-slate of any extent oc- 

 cur in England. I noticed a micaceous rock, which may be consid- 

 ered as an imperfect kind of mica-slate, near the granitic rocks of 

 Mount Soar Hill; but it was covered by wood, which concealed its 

 junction with other rocks. On the western side of Anglesea, near 

 Holyhead, there are numerous rocks, of an intermediate kind, be- 

 tween mica-slate and talcous slate. The laminas are separated by 

 very thin seams of quartz ; and I observed some of them bent and 

 contorted in various directions, as is not unfrequently the case with 

 mica-slate in other districts. 



The mica-slate on the opposite coast of Ireland, near Bray, I am 

 inclined to consider as of the same formation with that in Anglesea. 

 Probably, this rock stretches under the Irish Channel, of which in 

 that parallel of latitude it may form the bed. The structure of both 

 rocks is the same, presenting the same divisions, by thin laminae of 

 quartz, but the mica of Anglesea is more combined with talc. Mica- 

 slate abounds in the Highlands of Scotland, and in many alpine dis- 

 tricts in Europe, particularly in the Pennine Alps. 



Gneiss and mica-slate are nearly allied to each other and to gran- 

 ite. Circumstances, attending the formation of granite, appear to 

 have produced a different arrangement of the component ingredients. 

 This is the more probable, as both gneiss and mica slate, sometimes, 

 graduate into granite, and have, at other times, a porphyritic struc- 

 ture. In some situations, the causes which change granite into gneiss 

 or mica-slate have not operated ; and we find neither of these sub- 

 stances separating granite from the rocks of the next class. 



An opinion has been advanced by Dr. Macculloch, that gneiss 

 and mica-slate have been deposited by water, though he admits the 

 igneous formation of granite : but granite is known, as before stated, 

 to vary much in the proportion and size of its constituent minerals 

 even in the same rock. Now, wherever the felspar was deficient, 

 and the mica and quartz abundant, or where the felspar was more 

 granular and the mica abundant the same process that formed gran- 

 ite in one part of a rock, would form gneiss or mica slate in another. 

 Every one who has examined the granit veine of the Alps, in situ, 

 will admit, that it had the same origin as common granite ; and again 

 they could scarcely hesitate to say, that gneiss and granit veine are 

 only mere varieties of the same rock, and must have had one com- 

 mon origin. The mica, in gneiss, is as much an igneous formation as 

 i that in granite, or in some volcanic rocks, 

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