262 J. Romanes, — The Peninsula of Nicoya, Coxta Rica. 



augite, analcime, and magnetite. The felspar is mainly labradorite 

 in well-formed latli-shaped crystals. The augite is colourless to pale 

 green, and is markedly granular. In all the slices examined there is 

 a considerable quantity of green uralite derived from the augite. 

 Analcime is generally present, either inter.stitially or in small 

 secondary veins. In a few instances the occurrence of this mineral is 

 curious, as it forms the central part of the felspar crystal, the 

 boundary between the two being perfectly sharp, although very 

 irregular. These dolerites are, I believe, also abundant, tliough 

 probably not to the extent of the finer-grained varieties, and simply 

 represent a deeper-seated phase of crystallization, both being derived 

 from almost identical magmas. In a specimen [9672] from the 

 gravel of one of the southern tributaries of the Hio Nosara the texture 

 is somewhat coarser and the augite tends towards the subophitic habit. 



One exceptional type [9673] remains to be described: the specimen 

 was collected from a large boulder in the llio Lapa where the rock 

 probably occurs in place. It is much coarser than any of the fore- 

 going varieties and approximates towards a plutonic habit. The 

 plagioclase felspar is very decomposed, but is distinctly more acid than 

 the usual labradorite, and builds large idiomorphic crystals; along 

 ■with this is some orthoclase. The chief ferromagnesian mineral is 

 a green augite with slight pleochroism, a variety lying between 

 common augite and aegirine augite ; secondary hornblende and 

 magnetite are very abundant. The rock has a distinct resemblance 

 to some of the monzonites of the Cerro Candelaria,^ and these latter 

 may quite easily represent an outlying part of the Xicoyan alkaline 

 province. 



4. The Nicoyan Series. 



I have given this name to a group of marine sediments which occupy 

 the lower ground in every part of the peninsula visited by me. These 

 deposits while varying considerably in lithological character preserve, 

 as a series, remarkable uniformity in widely separated localities. 

 They are well exposed in many of the stream sections near the village 

 of Nicoya, and form the flat expanse in which it is situated, but, as 

 previously remarked, wlierever the ground rises other rocks take their 

 place. The section along the bed of the Rio Marote may be taken as 

 typical. The deposits are always well bedded, and in the coarser 

 varieties form beds 2 to 3 feet in thickness. They strongly resemble 

 many of the Palaeozoic greywackes of Britain. The finer types have 

 the appearance of mudstones or shales. These alternating beds are 

 considerablj^ folded and occasionally dip almost vertically, although 

 gentle anticlines and synclines are more generally developed ; here 

 and there some faulting has taken place. The strike of the series is 

 very variable, but E. 30° N. is the normal. The same deposits 

 are exposed further down the Rio Marote at La Colouia, and there 

 again they occupy all the lower ground, but give way to igneous 

 rocks in the surrounding high ground. The beds exposed in the 

 river have a general strike of E. 35° S. with a vertical dip ; they are, 

 however, very much affected by minor contortions. 



1 Q.J.G.S., vol. Ixviii, pp. 110-12. 



