Vol. 68.] GEOLO&T OF COSTA EICA. Ill 



than the aiidesine, and has crystallized at a later stage, so that it 

 tends to form large plates completely or partly enclosing the lath- 

 shaped plagioclase-crystals. The ferromagnesian minerals are 

 represented by biotite and augite. The former is of a rich brown 

 colour, and intensely pleochroic. The augite is colourless, and 

 usually builds very irregular crystals ; although in a few cases 

 more or less idiomorphic individuals are present. The biotite and 

 augite have crystallized almost simultaneously, either together with 

 or just after the andesine. The accessory minerals are apatite and 

 magnetite. 



This rock shows a most striking similarity to the typical mon- 

 zonite of Monzoni, both in structure and in component minerals; 

 in particular the relationship between the orthoclase and the 

 plagioclase is identical with that which characterizes the type-rock 

 (see PI. IX, fig. 2). 



Boulders from Anonas. — These vary to a certain extent in 

 texture and mineralogical composition. They are slightly more 

 basic in character, but all show the same monzonitic structure as 

 those already described. 



All these types indicate a very close genetic relationship, and 

 might well have come from a single plutonic mass. There can be 

 little doubt that this mass must be exposed in the hills somewhere 

 above Escazu : probably at no great distance, to judge by the very 

 large size of the Escazu boulders ; while there is no agent except 

 water-action to account for their transport. 



A few specimens which I obtained from the Cerro Candelaria 

 strongly suggest, from their petrographical character, a hypabyssal 

 origin. About a quarter of a mile below the village of Patara on 

 the San Jose road, is a small exposure of rock which I think is 

 practically in place, although there are no other exposures near 

 it to indicate its relation to anything else. The fresh rock is green 

 and rather fine-grained, and weathers almost pure white. A micro- 

 scopic examination of this rock shows at once that it is of extreme 

 interest, as it is clearly allied to the plutonic rocks. In minera- 

 logical content and in structure it is similar to the normal 

 monzonites, but the texture is very much finer, and the rock might 

 well be named a micromonzonite. The plagioclase is oligoclase, 

 quite fresh and undecomposed. The orthoclase, as in the plutonic 

 type, is rather turbid and of later crystallization, partly enclosing 

 the plagioclase. A considerable quantity of free quartz occurs 

 interstitially to the felspar. The ferromagnesian mineral is a pale 

 green augite, in small more or less idiomorphic grains, sometimes 

 showing slight pleochroism. Magnetite is abundant, and a little 

 apatite is present. This rock is distinctly more acid than the 

 plutonic types, as regards both the plagioclase and the amount of 

 free quartz, and it seems probable that it may be of the nature of 

 an aplitic modification of the normal monzonite. 



Another type which strongly suggests a hypabyssal origin occurs 



