CHAP. 5.] UOCK FOKMATIONS. — UPPEK JURASSIC. 55 



showing a change to this extent within a distance of seventeen miles, 

 the general character of the fossils remaining the same. 



In no part of the Jurassics has it been found possible to trace out 

 narrowly any one group of beds for a long distance. This may be in 

 consequence of their lenticular accumulation in shallow water, evidence of 

 which is afforded by the prevalence of oblique lamination. The transition 

 from the lower part upwards is perfectly gradual and conformable, without 

 Conformity and condi- ^ break ; and there is nothing to suggest a change 

 tions of deposition. -^ ^j^^ ^^^^^^ couditions except the occurrence, 



mainly in the upper, coarser aud most cross-bedded portion, of layers 

 locally crowded with fossil terrestrial plants ; the beds themselves afford as 

 good evidence of old land denudation as any of the streams of the 

 district : their conglomerates point to shore deposits ; and while it must be 

 admitted that there is no mterminglijiff of marine with the terrestrial 

 organisms, it may still be possible that these latter were washed from the 

 land into estuaries and littoral regions of the Jurassic period which may 

 have been, from the prevalence of peculiar mineral salts or other reasons, 

 unfitted to support a marine fauna except in a few favoured localities. 

 This seems more likely than that a change took place from marine to 

 fresh water conditions without leaving some palpable traces in the 

 succession of the rocks to mark the alteration. 



Igneous intrusions have repeatedly found their way through the 

 Jurassic rocks. Their description belongs to another place, but their effect 



upon the rocks they traverse, ramifying sometimes 

 Igneous intrusions. ° 



with an intricacy that would defy expression, 

 except upon a map of very large scale, may be mentioned here. The 

 amount of alteration by the contact of these traps is various, gray shales 

 are turned to black and red or dark olive, highly altered trappean-looking 

 rock : sandstones generally become whiter and more silicious, resemblino- 

 quartzite ; while one peculiar white felspathic variety, in places where 

 overlying trap may have been denuded or where the association with 



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