[ 324 ] 



XXIX. Proceedings of Learned Societies. 

 CxEOLOGICAL SOCIETY. 



[Continued from p. 207.] 



December 6th, 1911.— Prof. W. W. Watts, Sc.D., LL.D., M.Sc, 

 F.B.S., President, in the Chair. 



rPHE following communication was read : — 



' Geology of a Part of Costa Rica/ By James Bomanes, B.A., 



F.G.S. 



The paper deals chiefly with that part of Costa Bica which lies 

 to the west of San Jose as far as the Pacific coast. San Jose 

 itself is situated in a wide valley sloping gently westwards, and 

 drained by the Bio Grande and its tributaries (defined in the paper 

 as the San Jose Yalley). The northern boundary of this valley is 

 the chain of recent volcanoes which rise gradually from its floor, 

 while on the south the ground rises abruptly to form the Cerro 

 Candelaria. 



In this latter range of mountains are exposures of limestone, 

 marl, etc., together with various igneous rocks. The limestone is 

 that which is referred to the Cretaceous by B. T. Hill ; but an 

 examination of several exposures has failed to produce any 

 Cretaceous fossils, while the occurrence in places of vast numbers 

 of Balani points to a Tertiary age for the beds. As this limestone 

 stretches right across the Atlantic-Pacific watershed between 

 Cartago and San Jose, it yields clear evidence of an interoceanic 

 connexion in this area in Tertiary times. 



Of the igneous rocks, the most interesting feature is the presence 

 of many boulders of monzonite, indicating a plutonic mass in these 

 mountains. The surface of the San Jose Valley is composed of a 

 thick series of andesitic lavas into which the rivers have cut deep 

 gorges, exposing in some cases older river-deposits and buried 

 spurs of the Cerro Candelaria. On the Pacific coast at Barranca 

 and Manzanilla fossiliierous Tertiary beds are described. These 

 are all marine ashes, and in the Manzanilla district appear to rest 

 unconformably on an older limestone-formation. From the same 

 area a limburgite is recorded. 



The dominant features of the Pacific shore-line are due to 

 drowning, instead of to uplift as is the case on the Atlantic coast ; 

 these features have, however, been considerably modified by 

 subsequent alluvial deposits. 



The ' boulder-clays ' of Costa Bica are shown in many cases to 

 be normal river-deposits, though locally landslides and spheroidal 

 weathering have played an important part. These deposits in the 

 valley of the Bio Beventazon point to several distinct phases of 

 river-action. 



