s 



Yol. 68.] GEOLOaY op COSTA RICA. 119 



One other exposure is worthy of record. The road down the 

 creek to the lower end of the tunnel cuts through some fossiliferous 

 deposits. These are highly-weathered ashes of a deep rusty brown, 

 and contain a considerable amount of iron and phosphate. The dip 

 here is 30^ northwards, which carries the beds underground ; but, 

 if this dip be continued, they ought to be exposed again near the 

 bottom of the gorge ; and so they are probably the same as the 

 fossiliferous ashes which crop out along the river. The most 

 characteristic fossil of this weathered bed is a Pecten which Mr. 

 E.. B. Newton has kindly examined ; he says that it is very closely 

 allied to P. liemijhilli Gabb. It is also somewhat similar to the 

 casts of Pecten obtained from the marl above El Higuito. 



Fig. 3. — Section across the gorge of the Rio Virilla at El Brazil, 

 showing the probable relation of the fossiliferous ashes to the 

 lava-Jiow. 



[a=FossiIiferous ashes ; 6=Lava.] 



The general succession of the El Brazil beds appears to be as 

 follows : — 



Lava-flow. 



Yellow marls, comparable wihli those of Tres Rios. 



Coarse weathered agglomerate. 



Fossiliferous ashes. 



Massive agglomerate. 



On the north side of the river-gorge, opposite the intake-station, 

 the sedimentary rocks can be seen cropping out horizontally under 

 the lava-cap, which is here very thin, but thickens rapidly on 

 either side. There can be no doubt that these sediments formed 

 a spur of the Cerro Candelaria, which has been buried under the 

 lava-flow and subsequently dissected by river-erosion. Detailed 

 work here would probably throw some valuable light on the 

 succession in the Cerro Candelaria, as it is the only example of a 

 dissected spur that I saw, although no doubt others occur in less 

 accessible places. The rapid increase of dip in the beds, as they are 

 traced southwards from the river, is probably a very significant 

 fact. It suggests, at any rate, that the San Jose' Y alley consists of 

 a series of almost horizontal Tertiary sediments (marine ashes) 

 overlain by lava, and that southwards they pass laterally into 

 a region of intense folding, of which the Cerro Candelaria forms 

 the northern part. 



