J. Romanes — The Peninsula of Nicoya, Costa Rica. 261 



vast majority of those described from the mainland. Most of them 

 are dolerites which often carry analcime ; with them occur coarser and 

 finer grained varieties, but all show a remarkably close genetic relation- 

 ship ; from this area I do not possess one single specimen of an 

 andesite. This indicates that a sudden change has taken place in the 

 comparatively short distance separating the mainland from the part 

 of Nicoya which I visited. The dolerite series must have a wide 

 distribution if we are to judge by the number of boulders brought 

 down by all the rivers. 



Time and the difficulties of travel prevented me from making any 

 attempt at mapping even in the roughest way. One point, however, 

 which seemed clear was that round the villages of Nicoya and La Colonia 

 the igneous rocks came on in all cases exactly where the ground rose 

 sharply from the raised peneplain of the Nicoyan Series. I will 

 confine myself here to a description of the various types collected from 

 different localities. 



The finest-grained varieties are well exposed in the upper part of 

 the E,io Pila, west of Nicoya village, and they continue westwards 

 over the watershed down to the bottom of the next basin, which is 

 drained by the E.io Quiriman. These rocks are much jointed and 

 veined by calcite, and in the hand-specimen are very fine-grained and 

 non-porphyritic, with a dark, almost black colour. Examined under 

 the microscope [9666, 9667] they are seen to present some affinities 

 with the variolitic basalts, although without a chemical analysis it is 

 not easy to place them definitely. The dominant minerals are felspar 

 and augite ; the former is mainly labradorite, but with this occurs 

 some orthoclase. The felspars have crystallized in long acicular 

 crystals which often show plumose and stellate grouping. The augite 

 builds small stout prisms which in the sections give it a granular 

 appearance. Two kinds of interstitial matter are present ; a dark- 

 green glass with numerous rods and grains of magnetite, and a colourless 

 isotropic substance of very low refractive index which seems to be 

 analcime. This analcime is present both as interstitial patches and 

 as strings and veins, showing that to a certain extent this mineral 

 must be secondary, though much of it probably represents the final 

 stages of crystallization at a low temperature and with concentration 

 of mineralizing agents. Other zeolitic minerals with low double 

 refraction sometimes replace the analcime, but all the chief character- 

 istics of these finer- grained types recur in rocks with a very wide 

 distribution. In addition to the locality referred to above I have 

 collected similar rocks from the western coast of the peninsula in the 

 Playa Corillo, a bay opening to the Pacific just south of the Rio 

 Buenavista, from the gravels of that river and the Rio de Ora, and 

 also from the watershed above Buenavista, at a height of over 

 2,000 feet. One or two of these examples [9668, 9669] are distinctly 

 coarser in texture than the type described and appear to be in general 

 holocrystalline and show a transition to the coarser varieties which 

 are true dolerites and will now be described. They are best seen in 

 the hills to the west of La Colonia, especially in the gorge of the 

 Rio Lapa. The typical rock [9670, 9671] is even-textured, dark grey 

 in colour, and consists of a medium-grained aggregate of felspar, 



