112 ME. J. EOMANES OJs" THE [Feb. I9I2, 



as numerous boulders, in a small stream which crosses the road 

 above El Higuito. In the hand-specimen it is a dark rock, in 

 which felspar, augite, and pyrites may be easily distiDguished. 

 It is a typical olivine-dolerite, the olivine being represented 

 by pseudomorphs of carbonates. A curious feature of this rock is 

 the occurrence of pvrites in interstitial patches, often partly 

 enclosing the lath-shaped crystals of labradorite ; this pyrites has 

 been apparently the last mineral to crystallize. This may, of 

 course, be secondary ; but, if so, it is difficult to see what is replaced, 

 and there is no evidence in the slice of such an alteration having 

 taken place. 



The volcanic rocks which I obtained from the Cerro Candelaria 

 do not show any points of special interest. The majority are the 

 ordinary pyroxene-andesites so common in other parts of 

 the country. For convenience, the following description is given 

 as typical of the great majority of the pyroxene-andesites of the 

 country; subsequent descriptions will, as far as possible, only refer 

 to the more important variations from this type. The andesitic 

 lavas are generally rather dark, often mottled, and more or less 

 vesicular. The hand-specimens show well-formed crystals of 

 plagioclase and pyroxene in a fine ground-mass. Under the micro- 

 scope the felspar is seen to be labradorite, which builds large 

 tabular crystals generally strongly zoned and showing both albite 

 and Carlsbad twinning. The pyroxene is mainly augite, colourless 

 to pale green, and often with complex twinning. Some rhombic 

 pyroxene (hypersthene or enstatite) is occasionally present. The 

 ground-mass consists of small lath-shaped crystals of labradorite, 

 and granules of augite and magnetite in a glassy base. In those 

 under consideration a little red biotite is often present as an 

 accessory mineral, together with magnetite and pyrites. In certain 

 cases the structure is more of a doleritic type, the ground-mass 

 being a holocrystalline aggregate of minute lath-shaped felspars. 

 In one specimen of this type glomeroporphyritic strncture is well 

 marked; the rock contains aggregates of felspar- crystals showing 

 distinct plutonic structure. The felspar in these cases is principally 

 orthoclase. In the same specimen there is a curious secondary 

 deposit of quartz and hornblende, the former containing long 

 needles of the hornblende. 



In a stream-section above Escazu are some very thick beds 

 of an almost black fine-grained rock ; in the hand-specimen are 

 seen a few fragments of felspar and a little pyrites in a struc- 

 tureless ground-mass. A microscopic examination of the rock 

 shows that it consists of broken crystals of felspar and some 

 pyrites in a fine matrix of felspar and magnetite -dust. The 

 felspar, which is absolutely fresh, is chiefly orthoclase, with 

 Carlsbad twinning, and some plagioclase. The rock is un- 

 doubtedly a fine-grained volcanic tuft', and such rocks probably 

 have a wide distribution in these mountains, as many of the 

 boulders in the streams appear to be of a similar nature. 



