Tertiary Deposits in JSfortJi- Western Peru. 949 



local derivation, the sands of alien derivation.- The formation pre- 

 sents a number of unusual characters, among which are a total 

 absence of lamination and a singular vertical cleavage. From its 

 fauna, structures, and composition, but especially from its physio- 

 graphical relations, it is clear that the formation cannot be of 

 .aqueous origin. The breccias are manifestly local talus. The 

 distribution of the sands and loams, and their peculiar structural 

 characters, can, it is argued, be accounted for only by seolian action. 

 The winds, banking them up under the lee of southern slopes, must 

 have been from the north ; and, as the land seems then to have 

 stood sufficiently above its present level to lay dry the bed of that 

 part of the Bristol Channel, it is suggested that the alien sand 

 was blown from the older Pleistocene deposits of that hollow. 



A comparison is made with the loess, and it is shown that, out 

 of 20 characteristics, palseontological, structural, and otherwise, 

 the Clevedon drifts have 17 in common with that formation. 

 Those which they have not in common are assignable to the prox- 

 imity of Clevedon to the sea. The deposits are ascribed, accordingly, 

 to ' Loess-conditions ', acting in a region where local circumstances 

 appear to have been especially favourable. 



December 15th.— Mr. E. D. Oldham, F.B.S., President, 

 in the Chair. 

 The following communications were read : — 

 1. 'Structure and Stratigraphy of the Tertiary Deposits in 

 North- Western Peru.' By Thomas Owen Bosworth, D.Sc, M.A., 

 P.G.S. 



The westernmost ranges of the Andes, in the north of Peru, are of 

 pre-Tertiary age. The Eocene Pacific Ocean lay at the foot of them. 



The Tertiary rocks occupy a narrow strip of country between 

 the mountains and the sea. They are exposed in those areas 

 which have been denuded of their horizontal cover of Quaternary 

 deposits. The Tertiary consists of 15,000 to 25,000 feet of clay- 

 shales and sandstones, with innumerable thin seams of beach-pebbles 

 and shells. Thus, during the Tertiary Period, a large subsidence 

 was in progress. 



The strati graphical succession is as follows : — Feetm 



Miocene. Zorritos Formation 5000 + 



[ Lobitos Formation 5000 -f 



Eocene ' Ne ^ itos formation. 



J Clavilithes Series 1 -0004- 



(_ \ Turritella Series J 



The Tertiary accumulation is greatly broken up by intense 

 hlock-faulting ; between the fault-blocks are differential dis- 

 placements of many thousand feet. It is inferred that, in the 

 interval between the Tertiary and the Quaternary Periods, an 

 important movement occurred along a great fault-belt parallel 

 with the Andes. The mountains were further upraised, and the 

 sea-floor subsided to a great depth. 



The uplifting of the mountains caused a strip of territory along 

 the west side of them, 20 miles wide, to emerge from the sea. 

 This is the littoral : it was part of the crush-belt of the great fault. 



