260 J. Romanes — 'The Penimula of Nicoya, Costa Rica. 



2. Limestones and Cherts. 



These are, I think, the oldest rocks exposed in the peninsula, and 

 will probably be found eventually to have a wide distribution. Tlie 

 limestone is well seen about two miles north of the village of ISTicoya, 

 where it forms a conspicuous hill along the base of which runs the 

 road to Humo. The rock is fine-grained and almost white, but 

 with numerous joint-faces somewhat stained with iron. Chemical 

 tests show that the limestone contains some magnesia. Little 

 structure is seen under the microscope [9662],^ as considerable 

 recrystallizatiou has taken place ; there are, however, slight traces 

 of organic structure which point to the rock having originally been 

 a foramiuiferal limestone. This limestone hill rises sharply from the 

 very level ground which surrounds the village of Nicoya in precisely 

 the same way as the hills of older limestone from the Manzanilla- 

 Avangares peneplain of the mainland. This, coupled with the general 

 similarity in appearance of the deposits, is strong evidence in favour 

 of their belonging to the same series. I believe that many similar 

 limestone hills are to be found, especially to the north in the 

 neighbourhood of Humo ; and as in places I have seen the rivers 

 depositing calcareous sinter, limestone is apparently widespread. 



The other deposits which I am inclined to include in this series are 

 the lladiolarian cherts, although I quite admit that there is no direct 

 evidence to show that they belong to the same period. These cherts 

 I have only found in place in the upper part of the Rio Marote, 

 i.e. north-west of the Humo road ; but there, at least, splendid 

 exposures are to be seen in the river bed over a considerable distance. 

 Unfortunately the junction of the cherts with any other rocks is 

 nowhere exposed. Lower down the stream, but separated by 

 a considerable distance of river gravels, are exposures of the Nicoyan 

 Series, while up stream I was unable to find any rock in place above 

 the cherts. As will be seen from a subsequent part of this paper 

 fragments of these Radiolarian deposits occur plentifully in the 

 Nicoyan Series, which must therefore belong to a later period. The 

 cherts are very hard and compact and vary from red to green in 

 colour, and show much quartz veining on a minute scale. In thin 

 slices [9663, 9664] these rocks are distinctly disappointing ; they are 

 evidently rich in Radiolaria, but none of the fine structures are 

 preserved. They show an aggregate of cryptocrystalline silica the 

 Radiolaria being represented by circles of clear microcrystalline quartz. 

 A curious rock-type [9665] occurs plentifully in the Rio Nosara 

 region which may also be of the some nature. It is of a rusty-brown 

 colour with streaks and vesicles of soft iron oxides ; the slice of this 

 type certainly suggests affinities with the true cherts. 



3. The Igneous Rocks. 



Igneous rocks contribute very largely to the formation of ISTicoya, 

 and these form an extremely interesting branch of the geology. All 

 the rocks which I was able to collect differ greatly in type from the 



' The numbers in square brackets refer to slices in the Sedgwick Museum 

 collection. 



