46 Reports and Proceedings — Geologists Association. 



The occurrence of a Lamellibruucli fauna at the top of the Seminida 

 Zone is recorded as a feature of special interest. 



2. "Geology of a Part of Costa Eica." By James Eomanes, B.A., 

 F.G.S. 



The paper deals chiefly with that part of Costa E.ica 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 

 tlie Rio Grande and its tributaries (defined in the paper as the San Jose 

 Valley). The northern boundarj' 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 

 w})ich is referred to the Cretaceous by E,. T. Hill; but an examination 

 of several exposures has failed to produce any Cretaceous fossils, while 

 ihe 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 waterslied between Cartago and San Jose, it yields clear 

 evidence of an inter-oceanic connexion in this area in Tertiarj- 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 

 fossiliferous 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 hj subsequent alluvial 

 deposits. 



The ' boulder- clay s ' of Costa Rica 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 Rio Reveutazon point to several distinct pluises of river- 

 action. 



II. — Geologists' Association'. 

 December 1, 1911.— Mr. William Hill, F.G.S., President, in the Chair. 



Dr. A. Smith Woodward read a paper on a maxilla of Trtoonodon 

 ferox discovered by Mr. John Newton in the Purbeck Beds of Swanage. 

 The fossil was obtained from the ' Cap Stone ', about 26 feet above 

 the well-known Middle Purbeck mammal-bed, and just below the 

 ' Feather ', in which the only known jaw of 2'riconodon major was 

 found. The fourth premolar and the three molars are perfectly 

 preserved, and the third molar is shown to be reduced in size, with 

 only two cusps. 



