33A Prof. A. C. Ramsay on Geological Ayes 



<( On the Comparative Value of certain Geological Ages (or groups 

 of Formations) considered as items of Geological Time." By A. 

 C. RAMSAY, LL.D., Y.P.R.S. Received December 16, 1873*. 



There are several methods by which attempts have been made to esti- 

 mate the value of minor portions of geological time, one of which is 

 founded on calculations of the probable age of deltas, deduced from 

 estimates, more or less accurate, of the quantity of matter annually car- 

 ried in suspension in rivers, in relation to the area occupied by, and the 

 thickness of, any given delta, such as that of the Mississippi. But as 

 none of these deltas are completed, and as it is unknown when, in the 

 course of terrestrial changes, such completion may take place, no one 

 can, as yet, successfully attempt to apply this kind of knowledge to the 

 amount of time that was occupied in the formation of any of the ancient 

 geological deltas, such, for instance, as that of the Purbeck and Wealden 

 area. 



Mr. James Croll has, with considerable success, attempted to measure 

 that portion of geological time which relates to the last great Glacial 

 epoch, founding his conclusions on astronomical data calculated back- 

 wards for a million of years ; but, as yet, the precise beginning of that 

 epoch has not, in my opinion, been shown ; and in the absence of precise 

 data respecting the number of local glacial episodes that may have pre- 

 ceded the last, and the complicated calculations that would be necessary 

 to measure these intervals, even if all these episodes were known, no 

 data are yet accessible for the application of Mr. Croll's method to the 

 greater part of geological time. 



There are other ways in which the subject has been approached, but 

 always, of necessity, with a total want of definiteness with regard to their 

 value in the measurement of time. The relative thickness of different 

 formations gives no clue, or only a very slight one, to the solution of 

 the question. Again, when in great and thick formations that spread 

 over wide areas, such as those of Silurian age, an upper part of the 

 series is found to lie quite uncouformably on the lower half, it requires 

 but little experience in geology to infer that the unconformity indicates 

 a long lapse of unknown time, unrepresented by strata over a given area. 

 When we link such phenomena of striking unconformity with the disap- 

 pearance in that area of some of the genera and most of the species in the 

 older strata, and their replacement by new and, to a great extent, generally 

 of closely allied forms, this addition to our data gives no clear help in the 

 absolute measurement of time; for no one as yet has even dared to speculate 

 on the length of time that may have been necessary for the production 

 of results so remarkable as those deduced from the theory of evolution. 



I am well aware of much that may be said on the other side of this 

 particular question, such as that the incoming life of the later epoch 

 * Bead Jan. 29, 1874. See ante, p. 145. 



