Vol. 68.] GEOLOGY OP COSTA RICA. 129 



presence of several hills which rise sharply and suddenly out of the 

 plain, exactly like islands dotted over the surface of a sea ; these 

 are of no great height, probably reaching some 400 or 500 feet above 

 the general level. They at once suggest the peaks of an older 

 landscape, buried by later deposits. Beyond the plain stretch the 

 waters of the Gulf of jSTicoya, with the high mountainous peninsula 

 of Nicoya forming the background. 



A traverse across this country from the mines to the coast 

 reveals some points of interest, although the nature of the ground 

 prevents detailed observation during a single journey. As might 

 be expected, it is impossible to realize the existence of the plain 

 when actually crossing it, for it is everywhere deeply cut into by 

 rivers and streams, while the dense forest-growth shuts off any 

 distant view. Por the first half of the journey, the total distance 

 of which is about 14 miles, the few exposures seen are certainly in 

 the Manzanilla Beds, the deposits being very similar to those 

 described at Las Juntas, though somewhat darker. Dips and 

 strikes noted were, first, east 30° south, dip 15° northwards ; and 

 farther west the strike was east 50° south, and the dip 25° north- 

 wards. Approximately half-way to the coast strikingly different 

 rocks are encountered. These consist of very fine, compact, cream- 

 coloured limestones. So far as can be seen, they occur on the 

 lower slopes of the hills which rise from the plain, and it is highly 

 probable that these hills are formed entirely of limestone or at least 

 of rocks belonging to the same series as the limestone. In these 

 rocks are preserved numerous organic remains, especially foramini- 

 fera belonging to Rotalia and allied genera, together with some 

 badly-preserved corals and calcareous algae of the nature of 

 jN'ullipores. 



An interesting feature is the occurrence of some felspar and 

 fragments of cryptocrystalline silica, giving evidence of contem- 

 poraneous volcanic action. 



Somewhat farther west, beside some native huts, rises a low 

 mound composed of a very dark, basic, igneous rock. This is a 

 limburgite entirely free from felspar, and is a type which I 

 believe has never before been recorded from Central America. In 

 the hand-specimen the rock is dark green, owing to the serpentine ; 

 while grains of fresh olivine may be seen, and numerous bronzy 

 crystals of rhombic pyroxene are visible. The rock has originally 

 been composed of roughly equal quantities of olivine and enstatite, 

 with a comparatively small amount of glassy ground-mass which 

 is crowded with small cr5'stals of augite, enstatite, and magnetite. 

 The olivine, which forms large rounded crystals, has been largely 

 serpentinized ; and only in places are fresh grains of olivine seen 

 scattered through the serpentine pseudomorphs (see PL IX, fig. 3). 



Many rounded patches of colourless or pale-green serpentine are 

 present, which seem to represent infilled vesicles. The rhombic 

 pyroxene builds tabular or lath-shaped crystals which show well- 

 developed prismatic cleavage, with extinction parallel to this. The 

 colour is very pale pink, but no pleochroism can be seen. The 



Q. J. G. S. ^^o. 269. K 



