126 MR. J. EOMAKES ON THE [Feb. I912, 



to describe tbein in detail withont careful mapping would involye 

 useless repetition. The majority are basic pyroxene-andesites, 

 some containing only augite, Trhile others carry hyperstliene in 

 addition. Some of these rocks appear to form large dykes, as, for 

 example, a very dark rock, collected at the lOth-level tunnel, 

 which forms a great band some 400 feet wide apparently cutting 

 through the other lavas ; however, even where the field-relations 

 of the rocks show them to be hypabyssal, the microstructure is 

 essentially volcanic. Many of these lavas show very perfect 

 columnar structure, as, for example, on the banks of the Eio 

 Avangares about half a mile below the mines. I noticed here 

 intercalations of the same volcanic conglomerate as that which 

 has been recorded from El Brazil and Anonas. The boulders 

 forming the conglomerate are of a very vesicular augite-hypersthene- 

 andesite, the vesicle being in almost every case lined with a thin 

 coating of a brightly polarizing mineral, showing the initial stages 

 of infilling. 



There is one type of lava from Avangares which merits descrip- 

 tion, as it is quite unlike any other lava that I have seen in the 

 country. The specimen was collected from a large angular block 

 in the railway-cutting between Avangares and the Boston Mine ; 

 and, although it was unfortunately not in place, it cannot have 

 been transported far. In the hand-specimen the rock is of a bluish- 

 green colour, with well-formed black crystals of hornblende 

 scattered through it. The ground- mass is cryptocrystalline or 

 glassy, and has undergone some silicification and calcification. The 

 felspar is labradorite in large porphyritic crystals, showing consider- 

 able alteration along the cleavage-lines; this has gone on in 

 certain cases to such an extent that merel)' isolated unaltered 

 kernels of felspar remain. Hornblende is well developed ; this 

 specimen, together with one to be described later, forming a con- 

 glomerate at Manzanilla (see p. 130), and the boulders from Cartago 

 (see p. 115) comprise the only hornblendic rocks that I have 

 collected in Costa Eica. The hornblende in this lava is olive-green, 

 with sometimes a brownish tinge; it shows well-developed prismatic 

 cleavage and strong pleochroism : the extinction-angle is about 12°. 



A short distance north of the mines coarse volcanic agglomerates 

 are largely developed. These are very compact, greyish- green, 

 and contain large angular fragments of lava. Little or no bedding 

 is visible, and they probably represent the site of one of the vents 

 through which the lavas and ashes were erupted. 



The auriferous quartz- veins in this district have a general 

 parallelism, striking about north 20° to 30° east. Many of them 

 occur along lines of intense brecciation, and some of^i^e veins 

 represent crush-bands recemented by quartz, so that fragments of 

 the ' country-rock' are scattered right through the vein. A slice 

 cut from a specimen of the very rich Tres Hermanos vein shows 

 the quartz to be in a microcrystalline or cryptocrystalline state : 

 along with the quartz very numerous small crystals of pyrites are 

 visible : these latter carry the gold. The formation of these veins 



