Petrology of the Lavas. 333 



generation of augite, of melilite, apatite, primary analcite, a 

 little biotite, and some spinellid, probably both magnetite and 

 perovskite. 



The augite phenocrysts are fairly strongly colored, yellowish- 

 green and zoned, the outer layer darker with purplish shade, 

 such as is usually associated with a higher percentage of titanium. 

 Olivine is completely decomposed and represented by pseudo- 

 morphs in serpentine, calcite, and a pleochroic (bright green to 

 yellow) micaceous mineral. Its size is about that of augite. 



The groundmass consists of narrow elongated crystals of 

 brownish augite forming a complicated lattice whose interstices 

 are filled with colorless melilite with typical cross-striation and 

 " pflockstruktur." It is roughly tabular to the base, but seldom 

 shows crystal outline. Apatite is abundant in slender, well- 

 formed prisms. 



Perovskite is inferred to have been present in the undecom- 

 posed rock, from the occurrence of brownish, almost opaque, 

 octahedral crystals which seem to have been partially changed 

 into a granular aggregate or titajrite. Small, clear patches with- 

 out definite outline consist of analcite which appears to be 

 original. The melilite, when in close proximity to these patches 

 or projecting into them, takes an idiomorphic habit and occurs 

 as well-formed tables flattened parallel to the base. A micro- 

 photograph of the rock is seen in fig. 1. 



Lamprophyric and Moncliiquitic Hocks. — It is probable that 

 all the rocks which fall under this heading might be classed with 

 the monchiquites in a broad sense, the essential constituents being 

 augite, biotite, and analcite, with apatite as an important acces- 

 sory. Olivine appears absent in all cases. Augite and biotite 

 are interchangeable, with all gradations between all augite and 

 all biotite. The various types, therefore, find their parallels in 

 the fourchite and ouachitite of Williams and Kemp. 



The best fragments of these rocks come from the depth of 1140 

 feet. They are moderately fresh, compact, gray rocks without 

 phenocrysts, consisting of elongated crystals of a brownish and 

 purplish augite and deep red biotite in a somewhat decomposed 

 base. This base consists of calcite and analcite, the latter often 

 in clear areas, and is traversed in all directions by slender needles 

 of apatite. A photograph of this type is seen in fig. 2. 



Rocks of this character have been met with at 1002-1022 and at 

 1045-1075 feet, in the latter almost free from augite, and they 

 were the most prevalent in the upper 100 feet of the igneous 

 rocks, though much decomposed. 



Keratophyre. — Th.Q only distinctly feldspathic rock found 

 comes from the higher portion of the well. A section from a 

 fragment in the sample from 550-554 feet proved the rock to be 

 allied to the keratophyres, since it consists of a fine-grained 

 aggregate of albite and microperthite enclosing a few larger 

 albites, rare flakes of biotite, and fairly abundant needles of 

 apatite." 



