110 MR. J. eomajStes on tde [Feb. 191 2, 



below the quarries the road cuts through a pale sandstone precisely 

 similar to that described above. A short distance farther down a 

 deep-yellow marl is exposed, showing very clear bedding (strike 

 €ast and west ; dip 45° northwards). 



This deposit yielded no fossils, but exhibited a very remarkable 

 resemblance to a yellow marl, to be mentioned later, in the gorge 

 of the R'lo Yirilla at El Brazil. Lying on the top of the road- 

 embankment here was a huge I'ectangular mass of quartzite, about 

 10 feet square and some 2 feet thick. It was quite unrounded, 

 and had probably suffered little transport. 



» 



{b) The Igneous Eocks. 



Exposures of igneous rocks are not plentiful in the Cerro Can- 

 delaria, and those that do occur are by no means easily accessible. 

 I was able, however, to collect a number of boulders, some of which 

 cannot have travelled far, and a study of these gives a very fair 

 indication of the rock-types which a more detailed examination of 

 the range may eventually reveal. 



The most interesting rocks are the plutonic types, and, so far as 

 I am aware, these have never been previously described. In the 

 village of Escazu, about 4 miles south-west of San Jose, there are 

 several large boulders lying by the roadside. These must weigh 

 several tons each, and consist of an even, coarse-grained, f^Tey, 

 syenitic rock. The same type of rock is found in the gravels of 

 the Tiribi at Anonas, and farther down the valley in the gravels of 

 the Kio Virilla at El Brazil. As plutonic rocks are of very rare 

 occurrence in the sierras of Costa Eica, a rather full description of 

 these will perhaps not be nnwelcome. 



The Escazu boulders. — The hand-specimen shows a coarse- 

 grained, even -textured, plutonic rock of a greenish-grey colour. 

 Upon examination with a lens, it is seen to consist largely of 

 felspar, together with brown biolite and some augite. The felspar 

 is partly fresh and colourless, and partly stained and pale vellow. 

 The fresh felspar shows very distinct albite-twinning, while the 

 biotite occurs in characteristic lustrous brown plates. Under the 

 microscope the rock is seen to consist of quartz, felspar, augite. 

 and biotite, together with several accessory minerals. The struc- 

 ture is coarse, but many of the felspars have too pronounced a lath 

 shape for the true granitic structure. Eree quartz is present, 

 though in small quantity ; it forms small irregular grains with 

 minute liquid inclusions. The bulk of the rock is made up of felspar, 

 two kinds being present. The most plentiful is a plagioclase in 

 well-formed lath-shaped crystals showing very narrow twin lamelljB 

 on the albite law, and in some cases pericline-twinning also. 

 The extinction- angle measured from the twin plane is 16°, while 

 the refractive index is higher than that of quartz, showing the 

 felspar to be andesine. It is quite fresh, and free from any 

 products of decomposition. The other felspar is orthoclase, which 

 is present in considerable quantity. It is much more decomposed 



