Introduction to Geology, 71 



and schists, or to beds of sand ; while the sandstones, losing 

 their cohesion, become siliceous clays, or friable sands; the 

 sands pass into chert, and the gravel into breccia, or conglo- 

 merates. Of this changeful property the red marl, or red 

 sandstone formation, is an instance. The lias, the cornbrash, 

 and the ferruginous sands, afford numerous examples of a 

 similar tendency. 



It must not be inferred that these deposits, to which we shall 

 hereafter more specifically advert, present an absolute uni- 

 formity of character in every part of the world, or even that 

 they can be at all times satisfactorily identified at remote 

 points. Modifications of those characters by which they are 

 best distinguished within the limited area of our island, enor- 

 mous expansions in some cases, the introduction or the absence 

 of certain beds or members in others, increase the difficulty of 

 the investigation. At such times the observer has recourse to 

 the organic remains, as a frequent means of removing his 

 doubts when most other circumstances are equivocal. 



Such is the diversity observable in the composition and 

 external features of the secondary formations, that the study 

 of this division is infinitely more attractive, and in its results 

 more singular than that of the primary order, and its mineral 

 products. " The knowledge of these substances, which is the 

 ultimate object of the mere mineralogist, is to the geologist 

 only a subordinate acquisition, and forms but the alphabet by 

 which he endeavours to decipher the chapter of nature which 

 he studies. Hence the rarer varieties which, in the estimation 

 of the mineralogist, possess the highest interest, will, in the 

 eyes of the geologist, attract the least regard." 



There is little doubt that the advances which have been made 

 in the developement of the secondary strata have been mainly 

 owing to the high interest which their extraneous fossils have 

 excited. A powerful stimulus has also been derived from the 

 writings of the celebrated French naturalist. Baron Cuvier, 

 whose discoveries have thrown a charm over this branch of 

 science, and strewed the path of all succeeding geologists with 

 flowers. "This study is rendered interesting," he observes, 

 "by the variety of productions of parti^-lor general revolutions 

 which it affords, and by the abundance of the different species 

 which alternately offer themselves to view ; it neither has that 

 dull monotony which attaches to the study of the primitive 

 formations, nor does it force us, like the latter, almost neces- 

 sarily into hypotheses. The facts with which it is conversant 

 are so })rominent, so curious, and so obvious, that they may 

 suffice to occupy the most ardent imagination ; and the con- 



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