114 



ME. J. ROMAjS^ES ox the 



[Feb. 191 2, 



m 



< 



i^ 



recent volcanoes, a certain amount of subsidence in their vicinity 

 might well be expected, and in the absence of any detailed and 

 accurate observations on this well-marked 

 feature, this seems to be the most plau- 

 sible explanation (see fig, 2). 



The valley between the two above- 

 mentioned mountain-ranges is drained by 

 the E,io Grande and its numerous tribu- 

 taries. These rivers all flow in deep 

 gorges, which often have almost perpen- 

 dicular sides. The result is, that in a 

 general view of the area from any of the 

 high ground surrounding it, the rivers 

 are quite invisible ; and the general effect 

 is that of a huge waterless valley sloping 

 gently to the west. Exposures in an 

 area such as this are naturally not very 

 plentiful ; but some idea of the geological 

 structure is obtained from a study of the 

 river-gorges, where there are some very 

 fine exposures. In every case the sur- 

 face of the ground seems to be occupied 

 b}' a lava-flow or flows of great extent, 

 and it is only well to the west of San 

 Jose that any deposits under the lavas 

 are exposed. It is in this area that 

 Mr. E. T. Hill ^ describes the occurrence 

 of ' bolsons ' or old lake-basins : one at 

 Cartago, another at San Jose, and a third 

 at Alajuela. The Cartago bolson is es- 

 pecially well marked, and the ground 

 there is evidently a great alluvial flat. 

 These lakes must have been caused by 

 inequalities in the surface-lavas forming 

 dams across the valley, and giving rise 

 to large but shallow sheets of water. 

 These would have but a brief existence, 

 as they would be rapidly silted up by 



the volcanic ash and mud brought down 



from the volcanoes, giving rise to flat 

 alluvial tracts covered by redeposited 

 volcanic ash, and such appears to be the 

 case in these areas. 



The evidence of a lava-dam is clearest 

 in the instance of the Cartago area. At 

 the eastern end of that bolson the ground 

 is covered by huge boulders of lava, 

 which are worked for building-stone under the name of ' Cartago 

 Granite.' These seem to have beeu generally regarded as transported 



1 Bull. Mi-s. Comp. Zool. Harvard, toI. xxviii (1898) p. 225. 



