Vol. 68.] GEOLOGY OF COSTA RiCA. 123 



Under the microscope the character of the rock is clearly seen : 

 the chief constituent is the purple glass, which forms the basal 

 portion of the flow: it is present in large irregular fragments which 

 may be distinguished one from the other by slight colour-differences. 

 These fragments of glass are singularly free from any crystals ; only 

 very occasionally are labradorite or enstatite found embedded in 

 them. The rest of the rock consists of a crushed mass of glass in 

 which are embedded crystals, often broken, of labradorite, augite, 

 and rhombic pyroxene. In addition to these occur fragments of 

 more normal andesites, which have been picked up by the lava 

 while still molten. 



Another good section of the lava is seen in the gorge of the Rio 

 Alajuela, a short distance above the junction of that stream with the 

 Rio Grande, where it overlies the old mudslides previously mentioned. 

 The rock here is of the brecciated type, and contains intercalations 

 of volcanic conglomerates exactly similar to those that occur at 

 El Brazil. The lava is also well exposed in the Rio Virilla near 

 its junction with the Rio Grande, and here it shows beautiful 

 columnar structure. This peculiar brecciated type of lava has also 

 been found in the lower part of the gorge at Anonas, and again near 

 the bridge over the Rio Virilla, on the road between San Jose and 

 Escazu. 



Westwards from Cebadilla, as mentioned before, the Ban Jose 

 Valley narrows between the northern and the southern mountain- 

 ranges. In places, however, patches of the old level surface of 

 the valley may be seen as broken terraces along the side of the 

 gorge. Still farther west the valley opens out again, into what 

 Mr. R. T. Hill speaks of as ' the San Mateo peneplain ' ; and this 

 appears to slope gradually down with no abrupt change of level to 

 the top of the cliffs along the Pacific coast, which attain heights of 

 100 to 200 feet. As far as is known, this plain, to within a short 

 distance of the sea, is occupied by lavas and ashes ; but time did 

 not permit me to investigate any of this area, except the part right 

 down on the coast. 



V. The Pacific Coast. 



(«j Barranca. 



In this and the following paragraphs attention will be confined 

 to a description of the rocks exposed in the localities visited, the 

 general description of the coastal topography being left for a later 

 section. The coast-line south-eastwards from the mouth of the Rio 

 Barranca provides many good exposures of rocks, which form bold 

 headlands and sea-cliffs. These are most typically seen on that 

 part of the coast where the new railway is taken along the shore, 

 close to the headland known as the Carballo. Here the cliff 

 presents a vertical face over 100 feet high, and in it are exposed 

 a great variety of sediments many of which are richly fossiliferous. 



