Vol. 6S.'] GEOLOGY OF COSTA EICA. lOT 



Passing eastwards, the next sections are seen in the foot-hills 

 immediately south of the town of Tres E,ios. Here again there are 

 two quarries, the one some 50 feet above the other, at an altitude 

 of about 4200 feet above sea-level. 



In the upper quarry the limestone is exactly similar in litho- 

 logical character to that of Patara ; but in the lower one it is 

 totall}^ recrystallized, and no trace of organic remains is visible. 

 The dip and strike in the two cases are about the same (strike 

 north 30° east, steep dip southwards), but the abrupt change in, 

 lithological character strongly suggests that a fault or thrust exists 

 between them. The weathering of this limestone is curious; it 

 is very impure, and so the leaching-out of the calcareous matter 

 leaves a loose sandstone-like material which cannot be recognized 

 as limestone, except by actually tracing the gradual change. 



The same type of rock is found in several quarries at Agua 

 Caliente, some 2 miles south of Cartago. Here, also, the lime- 

 stone is almost completely recrystallized. At this place, as the 

 name implies, there occur hot springs ; these rise through the 

 limestone, highly charged with calcareous matter which is deposited 

 round them as sinter. The bedding of the rocks here is very clearly 

 marked, the strike being north and south, with a gentle varying 

 dip to the east. An examination of this rock under the microscope- 

 shows an impure crystalline limestone, with a very few traces of 

 organic remains. The impurities consist of angular fragments of 

 quartz and a few small crystals of plagioclase, together with 

 numerous fragments of what appears to be a yellow volcanic glass,, 

 which in certain cases shows some devitrification ; thus it is evident 

 that, during the deposition of the limestone, volcanic action must 

 have been in progress at no very great distance. 



The exposures are too few and too wide apart to enable any 

 opinion to be formed as yet, as to the general structure of the belt 

 of limestone ; but there can be no doubt that it is intensely crushedy 

 folded, and faulted in such a way that the actual dip and strike 

 observed in the few isolated exposures are probably of little value. 



The limestone where not altered is richly fossiliferous, but rather 

 by virtue of an extraordinary number of individuals than by a 

 great variety of species. 



As mentioned by Mr. E-. T. Hill ^ the most conspicuous fossil is 

 a large species of Pecten, which occurs in all the exposures where 

 unaltered limestone is found. The larger specim.ens measure some 

 6 or 7 inches across ; they are almost equivalve, and ornamented 

 with strong and rather widelj'-separated ribs, . A smaller species, 

 averaging about 2*5 inches across, is also abundant ; it has the 

 ribs rather widely separated, as in the larger species, but is much 

 more convex. J^either of these species have I been able as yet to 

 identify with described species. These, together with some small 

 specimens of Ostrea, are the only visible fossils in the lower Patara 



1 Bull. Mas. Comp. Zool. Harvard, vol; xxviii (1898) pp. 226-27. 



