as items of Geological Time. 335 



may be merely the result of migration from some other area or areas, 

 where it lived contemporaneously with the forms imbedded in the older 

 strata ; but this by no means gets rid of the question of time, with those 

 who may believe in an hypothesis so uncertain, if it so happen that they 

 also uphold the doctrine of evolution. Looked at in this light, it is 

 obvious that the balance of probability is largely in favour of the greater 

 proportion of the specific forms in a new formation being, in the common 

 meaning of the word, of later date than those of an older formation, on 

 which the newer strata lie unconformably. 



Neither is the main question altered by the circumstance that a pro- 

 portion of Palaeozoic genera are, in some parts of the w r orld, occasionally 

 and unexpectedly found along with Mesozoic associates. The fact remains, 

 that changes in life have been produced, during lapses of time, in specific 

 and consequently in generic forms, and that such contrasts of specific, 

 and often of generic, forms are always most striking where marked un- 

 conformities are found of a kind which prove that the lower strata had 

 previously been much disturbed, and, as laud, had suffered much denuda- 

 tion before being again submerged. 



Seeing that speculations such as those enumerated, even when founded 

 on well-established facts, afford but little help in the absolute measure- 

 ment of geological time, it has occurred to me to look at the question 

 from another point of view, and, in a broad manner, to attempt to esti- 

 mate the comparative value of long and distinct portions of geological 

 time, all of which are represented by important series of formations. 



In two papers * I have attempted to show that the Old Bed Sandstone, 

 Permian, and New Red series were all deposited, not in an open sea, but in 

 great inland lakes, fresh or salt ; and this, taken in connexion with the 

 wide-spreading terrestrial character of much of the Carboniferous series, 

 showed that a great continental age prevailed over much of Europe and 

 in some other regions, from the close of the Upper Silurian epoch to the 

 close of the Trias. The object of the present memoir is to endeavour to 

 show the value of the time occupied in the deposition of the formations 

 alluded to above, when compared with the time occupied in the deposi- 

 tion of the Cambrian and Silurian rocks, and of the marine and fresh- 

 water strata which were deposited between the close of the Triassic 

 epoch and the present day. 



Partly for the same reasons that I consider the Old Eed Sandstone to 

 have been a lake formation, so I think it probable that the red and 

 purple Cambrian rocks of Scotland, Shropshire, and "Wales were also 

 chiefly deposited in inland waters, occasionally alternating, as at St. 

 David's, with marine inter stratifications, generally marked by grey slaty 

 f ossilif erous shales, somewhat in the same manner that several bands coii- 



* Quarterly Journal Geol. Soc. 1871 (vol. xxvii. pp. 189-198 & 241-254), " On the 

 Physical Relations of the New Eed Marl, Ehsetic Beds, and Lower Lias," and " On 

 the Ked Eocks of England of older date than the Trias." 



