360 CONDITIONS OF FOLIATION. 



The limestone of Teesdale is a hard rock, but where it touches the 

 basalt of that country it has become crystalline. The shales are also 

 altered (fig. 70, p. 337). The upper portions of the slate system in 

 Shropshire and Radnorshire, where that system is immensely thick, 

 show similar changes. 



Decreasing Effects of Pressure and Heat in Newer Strata. The 

 effects of general pressure and heat continually decrease among the 

 superior strata of the Saliferous, Oolitic, and Cretaceous systems, and 

 seem almost wholly lost in the tertiary strata of this country. It is 

 chiefly to this graduated effect of heat that we may ascribe the dis- 

 tinctness of the rocks in different parts of the series. Thus, to take 

 the calcareous rocks, we have a series gradually changing in proportion 

 to their antiquity, from crystalline limestone, through highly condensed 

 carboniferous limestone, to compact lias, concretionary oolite, marly 

 chalk, and tertiary lacustrine marls. Among sedimentary deposits 

 there is a series from gneiss through the hard sandstones associated 

 with the carboniferous limestone to the sands of the oolites, chalk, 

 and tertiaries ; and another series from cleavable slate, through jointed 

 greywacke slate, hard coal shale, compact red marl, lias and lower 

 secondary clays, gault, and clays of the tertiary period. There is 

 properly no sand, clay, or marl among the older strata. Indurated 

 shale, hard gritstone, and compact limestone are of rare occurrence 

 among the younger rocks. 



Effects of Heat in forming Granitoid Strata. Mr. Sharpe re- 

 garded the foliation of gneiss generally as due to metamorphic action 

 of the same kind as that which produced cleavage in slate, only more 

 prolonged and more intense, and this view is now generally accepted. 



These foliated rocks, which have the aspect of being derived from de- 

 composed granitic rocks, with subordinate and associated strata equally 

 devoid of organic remains, constitute, according to the testimony of 

 observers, the lowest geological group, and in most countries were origi- 

 nally water-formed deposits. From the effects of metamorphic agents 

 upon these rocks, their natural analogy to granite is sometimes so much 

 heightened as to cause some uncertainty in distinguishing between 

 them, and enforce a conviction that the distinction between them is 

 one of degree, and not of kind. The rocks of this whole series might 

 without impropriety be termed granitoid strata. 



Foliation. Foliation is certainly in most cases independent of 

 stratification, and Darwin observes that even when its direction cor- 

 responds to the strike of strata its dip is different. David Forbes 

 records at Crianlorich in Perthshire, beds of blue limestone resting on 

 mica schist, but with the limestone foliated by the development of 

 plates of mica so as to resemble gneiss ; and while the foliation in 

 the limestone appeared to be identical with the planes of bedding, in 

 the bed above the limestone the foliation is twisted and irregular. 

 At Jaegerborg near Christiansand foliated limestone abounds, and con- 

 tains patches of gneiss, besides being capped with gneiss, which is 

 itself capped with granite. 1 



1 Q. J. G. S., vol. xi. p. 167. 



