IMPERFECTION OF PAL/EONTOLOGICAL RECORD. 75 



or, at any rate, at a comparatively short distance from land. A con- 

 clusion so far-reaching as this requires, however, to be received with 

 the utmost caution ; the more so as it constitutes one of the most 

 weighty arguments in favour of the equally far-reaching conclusion, 

 that the present continental areas have been in the main regions of 

 elevation, and the existing oceans in the main areas of depression, 

 since the beginning of the Cambrian period, if not from a still earlier 

 period. That a large number of the known sedimentary rocks have 

 been formed from the wear and tear of pre-existing rocks, or have 

 been the result of the accumulation of the skeletons of animals and 

 plants, in comparatively shallow water and at moderate distances 

 from a coast-line, may be taken as certain. With our present im- 

 perfect acquaintance, however, with the nature and origin of many 

 of the older sediments of the earth's crust, it appears hazardous to 

 conclude that all the sedimentary rocks have been laid down near 

 land. In various parts of the stratified series we meet with deposits 

 which may be paralleled with the Foraminiferal ooze, the Radiolarian 

 ooze, the Diatom ooze, or even the Pteropod ooze of the present day, 

 though it may be admitted that these deposits may sometimes have 

 been formed in a comparatively shallow sea. Moreover, it cannot, with 

 our present knowledge, be safely asserted that we have no ancient 

 representatives even of the " abyssal clays " of the deep oceans of the 

 present day. On the contrary, it seems very possible that certain of 

 the sediments of such old systems as the Cambrian and Ordovician 

 were formed at great depths, and that they represent the modern 

 abyssal clays. This is particularly the case with some of the fine- 

 grained, red, brown, or green muds which occasionally form a con- 

 spicuous feature in the Cambrian and Ordovician series. Such 

 muds are not only singular for their extraordinary barrenness in fos- 

 sils, but there is good ground for thinking that they have been 

 formed by the decomposition of volcanic matter, while they com- 

 monly exhibit dendrites of manganese. 



IV. Disappearance of Fossils. — The last subject which need 

 be mentioned in connection with the imperfection of the palseontolo- 

 gical record, is that of the disappearance of fossils from rocks origin- 

 ally fossiliferous. This, as a rule, is due to " metamorphism "— that 

 is to say, the subjection of the rock to a sufficient amount of pres- 

 sure or heat to cause a rearrangement of its particles. When of at 

 all a pronounced character, the result of metamorphism is the more 

 or less complete 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 palaeontological record, and the irreparable 

 loss of much fossil evidence. 



Another not uncommon cause of the disappearance of organic 

 remains from originally fossiliferous deposits is the percolation through 



