144 Royal Society : — Prof. A. C. Ramsay on the 



of geological time, such as : — calculations intended to estimate 

 the age of deltas, founded on the annual rate of accumulation 

 of sediments ; the astronomical method followed by Mr. Croll, in 

 connexion with the recurrence of glacial epochs; the relative 

 thicknesses of different formations ; and the relation of strong 

 unconformity between two sets of formations in connexion with 

 marked disappearance of old genera and species, and the appear- 

 ance of newer forms. Having shown that none of these methods 

 give any clear help in the absolute measurement of time in years 

 or cycles of years, even when founded on well-established facts, he 

 proceeds to attempt to estimate the comparative value of long por- 

 tions of geological time, all of which are represented by im- 

 portant series of formations. 



The author then alludes to the subject of two papers by himself, 

 given t to the Geological Society in 1871, on the Red Rocks of 

 England, in which he attempted to show that the Old Red Sand- 

 stone, Permian, and New Red series were all deposited 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 Silu- 

 rian epoch to the close of the Trias. He then endeavours to show 

 the value of the time occupied in the deposition of the above- 

 named formations, when compared with the time occupied in the 

 deposition of the Cambrian and Silurian strata, and of the marine 

 and freshwater strata which were deposited between the close of 

 the Triassic epoch and the present day. 



After alluding to the probable mixed estuarine and marine cha- 

 racter of the purple and grey Cambrian rocks of St. David's, it is 

 shown that the Cambrian and Silurian series may be massed into 

 three great groups : — first, from the bottom of the purple Cambrian 

 rocks to the top of the Tremadoc slates ; these being succeeded 

 unconformably by the second group, the Llandeilo and Bala or 

 Caradoc beds ; on which rest unconformably the members of the 

 third series, ranging from the base of the Upper Llandovery to 

 the top of the Upper Ludlow beds. — each unconformable break 

 in stratigraphical succession being accompanied by a corresponding 

 break in palseontological succession. 



These three great divisions are next shown to be comparable, 

 in the time occupied for their deposition, to the three divisions 

 of Lower, Middle, and Upper Devonian rocks, which are consi- 

 dered to be the marine representatives of the Old Red Sand- 

 stone ; and therefore it follows that the time occupied in the depo- 

 sition of the latter may have been as long as that taken in the deposition 

 of the Cambrian and Silurian series. This position is strengthened 

 by the great palaeontological differences in the fossils of the Upper 

 Ludlow and those of the marine Carboniferous series, which seem 

 to indicate a long lapse of time during which, in Old Red Sandstone 

 areas, no direct sequence of marine deposits took place. 



The next question considered is, what relation in point of time 



