116 MR. J. ROMANES ON THE [Feb. I9I2, 



{b) San Jose. 



A good exposure of the receut lavas is seen iu the gorge of 

 the Eio Virilla about 4 miles north of San Jose, ^here that river 

 is crossed by the Alajuela Railway . The gorge of the river is here 

 some 50 feet deep with vertical sides, and for about a quarter of a 

 mile down-stream there are more or less continuous exposures of 

 lavas which show in the hand-specimens a considerable variation in 

 structure and colour. The rock exposed direct!}- below the railway- 

 bridge is darkish, and has a mottled appearance; it corresponds 

 exactly with the type, except for the absence of rhombic pyroxene. 

 The other lavas of this gorge are petrographically very closely 

 allied to the above, but many are highly vesicular. 



About 2 miles west of San Jose the lavas are again well exposed 

 in the Eio Tiribi, at the Anonas Power-Station of the San Jose 

 Electric Light Company. Here they belong to a type very different 

 from those in the Rio Yirilla, as they are practical!}' non-porphy- 

 ritic, and all show a very markedly banded structure ; thin purple 

 and black lamellae alternate in a highly contorted fashion, evidently 

 a complex flow-structure. In places bands of a curious breccia 

 are intercalated with the lavas ; but, as these breccias are much 

 better developed at El Brazil, the description of them will be 

 deferred to a later page. A microscopic examination of these lavas 

 is rather disappointing, as they show extremely little structure. 

 They are largely glassy, consisting of minute slender crystals of 

 felspar, magnetite dust, and a brown micaceous mineral set in 

 a glassy ground-mass. This brown mineral, which undoubtedly 

 is closely allied to biotite, is scattered plentifully throughout all 

 the slides examined ; it often assumes a spherulitic structure, 

 and its general appearance strongly suggests a secondary origin. 

 In composition these rocks probably differ little from the normal 

 andesites. 



A similar rock may be obtained from a roadside exposure just 

 below Escazu. 



(c) El Brazil. 



El Brazil, where a new power-station is being erected for the 

 Electric Light Company, is on the liio Yirilla, immediately below the 

 junction of that river with the Rio Oruca, and lies about 7 miles 

 south-west of San Jose, The river is here flowing in a deep gorge 

 which cuts right through the lava-flow and down into the under- 

 lying sediments, and as this locality is just at the foot of the 

 Cerro Candelaria, I was able to see very clearly how the lava- 

 flow terminates against the rising ground. 



The geology here may be divided into two main parts : (i) The 

 overlying lava, and (ii) the underlying sediments. 



(i) The lav a- flow shows several points of interest. In the 

 first place, it has the appearance of a single great flow, possibly 



