Vol. 68.] GEOLOGY OF COSTA EICA. 117 



some 200 feet thick, and, unlike the previously described cases, it 

 presents extreme uniformity throughout. 



The upper surface forms the level plain of the San Jose valley, 

 which stretches away eastwards to the range of recent volcanoes. 

 The lower surface of the lava is extremely uneven, in places it is 

 70 or 80 feet above the bed of the river, while at other points the 

 gorge has failed to cut through it. The relation of the lava to the 

 sediments appears to be that of a great flow which has buried the 

 lower spurs of the Cerro Candelaria, filling up the valleys between 

 them with a solid mass of lava. The present river is now cutting 

 down through these buried spurs. Petrographically, the lava here 

 differs but slightly from those which are exposed in the Rio Yirilla 

 north of San Jose. It is a typical augite-andesite. A 

 noticeable feature about this rock is the very well-marked glomero- 

 porphyritic structure, which forms patches wherein augite and 

 felspar show subophitic intergrowth, and produce a coarse doleritic 

 structure. 



The lava-flow itself presents some curious features, especially as 

 regards the types of jointing. One of these types is well seen in 

 the cliff over which the cataract falls some distance below the 

 intake-station. The lava here occurs in great concentric shells, due 

 to a circular type of jointing, the diameter of the outer shells being 

 as much as 30 to 40 feet (see PI. YIII, fig. 1). This structure 

 I believe to consist rather of concentric cylinders than of spheres. 

 Unfortunately, it is not jDossible to investigate this exposure closely ; 

 but, so far as can be seen, the lava appears to be perfectly fresh, 

 and the concentric structure is not due to the spheroidal weather- 

 ing which such a structure might naturally suggest in a tropical 

 climate. Associated with this structure is a deposit which, at 

 first sight, looks like a river-gravel interbedded with the lava. 

 This, however, on closer examination is seen to consist entirely of 

 rounded masses of a very vesicular lava with little or no matrix ; 

 this I take to rej)resent the scoriaceous surface of the lava-flow 

 broken up and modified by incorporation in the flow when part 

 thereof was still liquid. 



It is a fact worthy of record that these scoriaceous intercalations 

 always occur in close proximity to the places where the lava shows 

 the most fantastic jointing. These intercalations must be due to a 

 sort of over-rolling of the lava, and this might well have given 

 rise to circular flow-lines which oil cooling have determined the 

 formation of this remarkable structure. 



Another noticeable type of jointing consists of a kind of hori- 

 zontal columnar structure, and this causes the lava to protrude 

 from the sides of the gorge in great bunches of massive blades. 

 These blades are roughly square in cross-section and generally 

 taper to one end. Along with this kind of jointing the scoria- 

 ceous intercalations are also found. This type of fragmental 

 deposit is also recorded from the Pio Tiribi, at Anonas, and from 

 the lavas exposed in the E,io Avangares just below the gold-mines. 



