106 :me. j. EOiTAXEs ox THE [Feb. 191 2. 



These mountains, situated almost in tlie centre of the isthmus, 

 afford many interesting geological problems, since there is a con- 

 siderable number of formations represented; and in them I have 

 been able to trace marine sediments to a height of at least 6000 

 feet above sea-level. 



From a study of the localities which I was able to visit, the 

 geology may be divided into three main parts : — (a) The sediments ; 

 (b) the volcanic rocks ; and (c) the plutonic rocks. 



(rt) The Sedimentary Series. 



This may again be divided into two main groups, according to 

 lithological character : — (i) Limestone, and (ii) marl and sandstone. 



(i) The limestone is described by Mr. K. T. Hill as forming 

 the foothills of the Cerro Candelaria (or Sierra Candella, as he calls 

 this range) and is named by him the San Miguel Beds. From 

 a somewhat brief description of its fauna he assigns it to the 

 Cretaceous Period, saying that it provides the only example of rocks 

 of this age throughout the isthmian region. 



This limestone, which I shall term the San Miguel Lime- 

 stone, apparently stretches continuously from the village of Patara 

 (some 9 miles south-east of San Jose) eastwards to Agua Caliente, 

 which lies about 2 miles south of Cartage, and thus crosses the 

 Atlantic-Pacific watershed a few miles west of Cartage. Exposures- 

 are only to be found where the rock is being quarried, otherwise 

 the formation is entirely concealed by vegetation. Good sections 

 are to be seen in the quarries of Patara, Tres Eios, and Cartage. 

 In these the limestone varies considerably in appearance, owing to 

 the different degrees of alteration which it has undergone. 



The least altered type is a hard, compact, bluish-grey limestone, 

 with a considerable amount of impurity, and largely composed of 

 fragmental organic remains the nature of which will be considered 

 more fully on a subsequent page. 



The Patani quarries furnish a good example of this type. In 

 the lower of the two quarries the limestone is seen to be very much 

 jointed, so that dip and strike are difficult to determine ; but it 

 seems to strike at about south 40^ east, with a dip of 40° north- 

 eastwards. The limestone itself shows coiisiderable recrystalliza- 

 tion, and slickensiding is seen not only along the joint-faces, but also 

 in the internal structure. The effects of tropical weathering are 

 well illustrated here. The limestone passes up into 8 or 10 feet of 

 loose material, in general appearance rather like some parts of the 

 Chalky Boulder Clay of Britain. The weathering first affects the 

 rock along the joints, until only scattered fragments of the weathered 

 limestone remain, and these finally disappear in the upper part of 

 the clay. Another larger quarry may be seen a little higher up 

 the hill ; and here the limestone presents much the same appearance, 

 but is perhaps a little more recrystallized and slickensided, while 

 the strike is east and west with a southward dip of 65°. 



