Vol, 68,] GEOLoay of costa rica. 115- 



masses, but I think that the evidence is against this supposition. 

 An examination of a large number of the boulders showed an extra- 

 ordinary similarity in lithological character. The only difference 

 that can be observed is, that while some are grey, others are of a 

 dull red: now, if any collection of transported boulders is examined, 

 for example, in the river-gravels, one of the most striking facts is 

 the great range in character of the rocks. Even when most of them 

 are petrographically similar, the range in colour and texture is very 

 great, and consequently it is difficult to understand why the Cartago 

 boulders, if transported, should show so little variation in character. 

 I have no doubt whatever that these huge boulders represent a 

 single lava-flow, which has weathered out in place ; and I believe 

 that it was this lava-flow which dammed back the water to form 

 the Cartago lake. Similar collections of boulders produced in the 

 same way may be seen on the surface in several places west of 

 San Jose, often forming distinct hummocks. 



I shall now pass on to a more or less detailed description of the 

 rocks met with in the area under consideration. 



(a) Cartago. 



The rocks collected near Cartago are mainly from the laro-e 

 boulders which have been mentioned above. The most plentiful 

 are those used for building purposes. The rock is grey or dull 

 purple, very rough to the touch, and exhibits a pronounced vesi- 

 cular structure and a slight banding due to the parallelism of 

 the felspar-phenocrysts. They differ mainly from the type described 

 (see p. 112) in containing corroded crystals of deep" red biotite, 

 while the augite is quite subordinate to the hypersthene. Glomero- 

 porphyritic structure is well developed, consisting of aggregates of 

 felspar and hypersthene, which simulate a fine-grained norite. 



Rocks collected about 2 miles west of Cartago, though of the 

 same andesitic character, show some well-marked differences. They 

 are hornblende-bearing andesites. The hornblende is of a rich 

 brown colour, strongly pleochroic with a dark resorption-border. 

 A very little fresh olivine is present. The most curious feature 

 of these rocks is the occurrence of numerous pseudomorphs after 

 some ferromagnesian mineral. These have generally a border of 

 magnetite, while fibres of magnetite sometimes extend inwards at 

 right angles to the external edges. The mineral composing them 

 IS colourless, with a high refractive index, strong double refraction, 

 and shows no cleavage. The form of these pseudomorphs suggests 

 original hornblende, but the absence of cleavage is against this. 

 The probability is that they represent original olivine, replaced 

 by a mineral of the nature of iddingsite. 



About 2 miles west of Cartago is a small quarry, on one of the 

 slopes rising from the alluvial flat. The rock here exposed is 

 decomposed almost beyond recognition, but it contains in parts 

 much microcrystalline silica; it is probably a silicified volcanic ash, 

 and may belong to the Cerro Candelaria series. 



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