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CHAPTER XXI. 



DISTUIBUTION OF GNEISS AND MICA SCHIST. 



In England, &c. The extent of country occupied by old gneiss 

 and mica schist with their associated rocks is enormous, and there 

 are few districts of large area where granite appears without being 

 surrounded by foliated rocks. The order of their succession and 

 their relative thickness are very uncertain. In some districts gneiss, 

 in others mica slate, in others quartz rock, is the only rock seen, and 

 that is immediately succeeded by clay slates. There are even cases 

 where the whole metamorphic system is wanting, and large areas of 

 granite are immediately invested by clay slates and limestones con- 

 taining organic remains. In England, for example, gneiss and mica 

 schist, crystalline limestone and quartz rocks are almost unknown ; 

 but in Ireland, and especially in Scotland, they are abundant, and 

 include many gradations of mineral composition, such as chlorite slate, 

 talc slate, hornblende slate, &c. The general order of succession 

 among the older primary strata in Scotland may be represented in 

 a diagram as in fig. 76, but it 

 must be remembered that all 

 the terms of the series are sel- 

 dom co-existent in the same 

 section. 



In Cornwall and Wales the 

 granitic rocks are almost univer- 

 sally succeeded by modifications 

 of clay slate, Anglesea alone 

 exhibiting a quartzo - micaceous 

 group below all the Cambrian 

 slates. In Cumberland the gra- 

 nite of the river Caldew is in- 

 deed covered by rocks having the Fig ?6t 

 character of gneiss, mica schist, 



dark hornblende slate (provincially called whintin), and chiastolite 

 slate, but their area and thickness are inconsiderable, and the latter 

 rock soon changes to clay slate. At a place called Martindale, at the 

 eastern foot of Caldbeck Fells, is a fine-grained variety of gneiss in 

 very thin laminae. Granite veins are rarely known to divide any of 

 the rocks of this region, except on a small scale between Skiddaw 



