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CHAPTER X. 



A RETROSPECTIVE VIEW OF CERTAIN GEOLOGICAL FACTS AND IN- 

 FERENCES. RELATIVE AGES OF MOUNTAIN RANGES.- — PRELIM- 

 INARY OBSERVATIONS ON THE SECONDARY STRATA. 



Before we proceed to the Upper Secondary Rocks, it may be 

 useful to review some of the leading facts stated in the preceding 

 chapters, and to nodce certain enquiries, which may naturally present 

 themselves to the mind of the geological student. It appears from 

 an examination of the crust of the globe, wherever it has been sci- 

 entifically explored, that there is an order of succession or superposi- 

 tion in the rocks of every country, which may often be traced over 

 a considerable extent ; and that in countries very remote from each 

 other, an approximation to a similar order is observable, except in 

 one class of rocks which are obtruded irregularly, and cover other 

 rocks without any determinate order of succession, as described in 

 the last chapter. The succession of the several classes of rock, — 

 the primary, transition, secondary, and tertiary, — may be regarded 

 as certain, where they occur together. Nor is the universality of 

 this succession affected by accidental disturbances, which, in a few 

 instances, have overturned beds of primary rocks, and thrown them 

 upon secondary strata. In such cases, the latter are thrown out of 

 their natural position, as much as when a block of granite is carried 

 ^by inundations, upon rocks of recent formations. The few cases in 

 which granite is described as rising through and covering secondary 

 strata require critical examination ; and geologists should be particu- 

 larly upon their guard, to avoid being misled by erroneous or fabu- 

 lous sections of foreign localities. See p. 65. 



The succession of the different members of any one class of rocks, 

 is by no means so definite as that of the classes themselves. Many 

 beds common in one country cannot be discovered in another, and 

 hence it may be difficult to determine what part of a series they 

 occupy. 



It is easy to conceive, that the cause or causes, whatever they may 

 be, which have formed certain rocks, have been limited in the extent 

 of their action, as we know to be frequently the case on a smaller 

 scale, where a stratum of sandstone, &c., after preserving its regular 

 thickness for several miles, becomes gradually narrower, till at length, 

 in the language of the miner, it wedges out, and the strata above and 

 beneath, come into immediate contact. In other instances, the rock 

 which is interposed between two well known and identical rocks, in 

 distant districts, is not the same in both : this may be frequently ob- 

 served among the secondary strata, which will next be described. 



