264 J. Romanes — llie Peninsxila of Nicoya, Costa Rica. 



Passing southward to the Pacific Coast about Bueuavista Bay we 

 have on the one hand types [9676, 9677] distinctly coarser than that 

 described above, while on the other are fine pale marls. Tlie chief 

 mineralogical difference between the forruer and that described from 

 the Bio Marote are : (1) the almost complete absence of quartz and 

 Badiolarian chert as constituents ; (2) the presence of numerous 

 fragments of andesites. The latter is a point of interest, since I noticed 

 a complete absence of these rocks throughout my journey in Nicoya, 

 a very striking difference from the mainland of Costa Bica, where they 

 are by far the most prevalent rock-type. Interstratified with these 

 normal marine ashes are found much finer deposits [9678], which 

 merit description. They present a very fine-grained, compact 

 appearance, and are most commonly of a grey or cream colour, showing 

 a considerable amount of recrystallized calcite. They are very 

 extensively developed in the cliffs at either end of Buenavista Bay. 

 In a thin slice they are at once seen to present close similarity to 

 globigerine ooze. The rock is largely calcareous and treated with 

 dilute acid is reduced to a paste probably composed of volcanic dust. 

 Glohigerina is abundant, and the shells are infilled either by crystalline 

 calcite or phosphate ; the latter is pale brown in thin slices and 

 corresponds to the mode of infilling described by Sir John Murray.^ 

 A beautiful example of this type [9679] outcrops on the north bank 

 of the Bio de Ora, about \h miles from the sea. The rock is some- 

 what darker than usual, but under the microscope is seen to consist 

 largely of Glohigerina and presents a striking resemblance to the Chalk 

 Bock of tliis country. These globigerine mudstones alternate quite 

 abruptly with the volcanic ashes and must represent the sedimentation 

 during periods of quiescence. 



6. Coastal Topography. 

 The coastal topography of Nicoya corresponds exactly with the 

 description of the north-east shore of the Gulf of Nicoya, which 

 I have already published.^ The inland ridges run out to sea as 

 headlands, and between these are sweeping sandy bays. Buena- 

 vista Bay is a pretty example of this. The sand is thrown up into 

 a distinct storm beach which has tended to pond back the river, with 

 the result that much alluvium is being deposited behind the barrier. 

 As on the mainland, large tracts of dark alluvial soil are to be seen, 

 which represent completely silted-up bays, so that while the main 

 coastal outline has been determined by a comparatively recent sub- 

 mergence it has been extensively modified by the deposition of 

 alluvium. Near Humo ver}?- large flat tracts of this nature are to 

 be seen. 



7. SUMMAKY AND CONCLUSIONS. 



The formations met with are divided into three well-marked series : 

 (c) The Nicoyan Series. 

 {h) The igneous rocks. 

 {a) The limestones and cherts. 



^ Eeport of the scientific results of the exploring voyage of H.M.S. 

 Challenger, 1873-6, Deep-sea Deposits, p. 393. 

 2 Log. cit., pp. 131-3. 



