IMPERFECTION OF PAL^EONTOLOGICAL RECORD. 69 



area were very different ; and, as a necessary result, certain 

 groups of animals flourished in certain localities, and were 

 absent or but scantily represented in others. In the deeper 

 parts of the area we have an abundance of Corals, with 

 Crinoids, and at times Foraminifera. In the shallower parts 

 of the area there is, on the other hand, a predominance of 

 forms which affect water of no great depth. Still there is 

 no difference in point of time between the deposits of differ- 

 ent parts of the area ; and in order to obtain a true notion 

 of the Lower Carboniferous fauna, we must add the fossils 

 derived from one portion of the area to those derived from 

 another. 



In many cases, however, we are acquainted with but one 

 class of deposits belonging to a given period. We may have 

 the deep-sea deposits of the period only, or we may know 

 nothing but its littoral accumulations. In either case it is 

 clear that there is an imperfection of the palaeontological 

 record ; for we cannot have even a moderately complete re- 

 cord of the marine animals alone of a particular period unless 

 we have access to a complete series of the deposits laid down 

 in the seas of that period. 



IV. DISAPPEARANCE OF FOSSILS. The last subject which 

 need be mentioned in connection with the imperfection of the 

 palseontological record is that of the disappearance of fossils 

 from rocks originally fossiliferous. This, as a rule, is due to 

 " metamorphism " that is to say, the subjection of the rock 

 to a sufficient amount of heat to cause a rearrangement of its 

 particles. When of at all a pronounced character, the result 

 of metamorphism is invariably the obliteration of any fossils 

 which might have been originally present in the rock. To 

 this cause must be set down many great gaps in the palaeon- 

 tological record, and the irreparable loss of much fossil evi- 

 dence. The most striking example which is to be found of 

 this is the great Laurentian series, which comprises some 

 30,000 feet of highly metamorphosed sediments, but which, 

 with one not absolutely certain exception, has as yet yielded 

 no remains of life, though there is strong evidence of the 

 former existence in it of fossils. 



Another not uncommon cause of the disappearance of 



