Vol. 68.1 



GEOLOGY OF COSTA RICA. 



125 



Good examples of the coarser and darker tj'pes were obtained in 

 a very fresh condition from the Barranca tunnel. The coarsest 

 type consists very largely of slightly-rounded, dark-green, isotropic 

 grains. These sometimes contain small lath-shaped crystals of 

 labradorite, and appear to be a very dark volcanic glass somewhat 

 decomposed. This forms the bulk of the rock, but isolated crystals 

 of labradorite and augite are fairly plentiful. The matrix is 

 calcareous, and has recrystallized into large crystals of clear calcite 

 which enclose the grains in poecilitic fashion. 



A rather finer type from the sam.e locality shows a ranch larger 

 proportion of plagioclase and augite-crystals, along with fragments 

 of cryptocrystalline silica and glass. This type of rock is fairly 

 rich in small foraminifera, and has yielded Glohigerina, liotalia, 

 and Orhulina. 



The finest types, which have the appearance of mudstones and 

 are generally by far the most fossiliferous, consist of the same 

 aggregate of minerals in a much finer state of division. 



These fossiliferous beds can be traced for only a short distance 

 inland, and they are soon replaced by a coarse, unfossiliferous, 

 and. unstratified volcanic agglomerate, which contains in places 

 enormous blocks of volcanic rocks. These agglomerates are well 

 seen in the numerous cuttings along the new railway between 

 Cascajal and Cambalache, and in no case have any traces of organic 

 remains been observed in them. 



One of the commonest and most characteristic fossils in the 

 Barranca deposits is a species of Venus, which appears to be identical 

 with V. meridionalis, recorded by Charles Darwin from Patagonia. 

 It is also closely related to V. lualli, from the Lower Miocene of 

 Trinidad, and to V. ehergeiu/ii Boese, recorded from the Pliocene of 

 Mexico. The following list gives approximate identifications of 

 the dominant fossils from these beds : — 



Mactrinula macescens Guppy. 

 Bosinia cf, orbicularis Edw. 

 dementia dariena Conrad, 

 Turritella cf. gatunensis Conrad. 



Tyrula reticulata Lara. 

 Solarium cf. viliarelloi Boese. 

 Area sp. 



This assemblage points to a Miocene age for these beds, and they 

 may easily prove to be more or less contemporaneous with some of 

 the fossiliferous ashes farther inland. 



[h) The Avail gares Mines — Manzanilla. 



The gold-mining district of Avangares lies on the south-western 

 borders of a vast expanse of volcanic rocks which stretches 

 unbroken eastwards to the Aguacate Hills and from there is 

 probably continued across the Bio Grande as the Cerro Candelaria. 



Igneous rocks.— The lavas in this district show little 

 difference from the andesites so common all over the country, and 



