Petrology of the Lavas. 337 



chemically. Some of the sample of crushed rock at 1045 feet 

 was first washed free from any adherent chlorides or sulphates 

 coming from sea-water. These were at first present. The 

 rock powder was then dried, ground fine, and boiled with very 

 dilute nitric acid, and the solution filtered. Some of this 

 filtrate on being evaporated yielded abundant gelatinous silica, 

 from melilite of course, for the most part. Other portions 

 tested for chlorine and sulphuric acid gave negative results, 

 indicating the absence of sodalite and hauynite. 



Feldspar j pseudoleucite. — Feldspar in the form of sanidine 

 has been found in one or two isolated fragments in the rock 

 powders, as at 980 feet. It was not observed in any of the 

 " rock sections " with any certainty, save in one case. The 

 reader must remember, however, that the largest u rock sec- 

 tion" is not above 3mm. in diameter, while most of them are 

 about 1mm. and with prevailing alteration and very dense tex- 

 ture the opportunities are not good. In the case referred to, the 

 section, which is one of the largest, contains a whitish circular 

 spot cut in half by the edge of the section, and about one and 

 a half mm. in diameter. This consists, for the most part, of 

 small, irregular, interlocked, granules of an alkalic feldspar 

 mixed with some nephelite. The determination is difficult on 

 account of the thinness of the section and the considerable 

 degree of kaolinization, but some of the particles have a 

 refractive index practically the same as balsam while others 

 are lower and biaxial. The whole appearance of this area is 

 similar to that of pseudoleucites from Brazil, Arkansas, Mon- 

 tana, and other places, as described by the writer and other 

 petrographers. The chemical analysis given beyond suggests 

 also the possibility of occasional leucite in these rocks. In 

 addition the area contains masses of calcite, grains of secondary 

 titanite whose grouped disposition indicates that they are 

 secondary after augite microlites, and much altered perovskite. 



Analcite. — The colorless isotropic areas with low refractive 

 index are inferred to be largely of this mineral since many of 

 them exhibit well the cubic cleavage which, as Evans* pointed 

 out, is one of the best characters in helping to determine it. 

 That it is not sodalite is shown by the chemical test mentioned 

 previously. The original nature of at least some of this anal- 

 cite has been mentioned by Dr. Thomas and referred to under 

 augite. 



Texture. — The rock is holocryptocrystalline, microporphy- 

 ritic in fabric and dopatic, the groundmass prismoid divergent, 

 its character being determined by the interlaced prisms of 

 augite whose interstices are filled by the anhedral felsic 

 minerals. 



* Quart. Journal Geological Society, vol. Ivii, p. 38, 1901. 



