Petrology of the Lavas. 339 



into a black decomposition substance. Masses and strings of 

 secondary granular titanite are seen in places. Iron ore, oli- 

 vine, and biotite appear to be entirely wanting in it, and, 

 although it is one of the largest sections, containing about 12 

 square mm. of surface, it contains no phenocrysts. Consider- 

 ing the amount of mafic mineral present, this rock would per- 

 haps classify best as an augitite, of alkalic affinities. It might 

 be remarked, however, in this connection that rocks which 

 have in many cases been classified as nephelinites, nephelite-, 

 and leucite-basalts, etc., have quite as large a proportion of 

 mafic minerals, so that if stress is to be laid upon the felsic 

 component, as in their case, the rock would be called an anal- 

 citite, or perhaps a monchiquite, in lava form. 



Another variety of this group, mentioned by Dr. Thomas, 

 contains no augite, but is composed of abundant small biotites 

 embedded in a felsic cement. The best example was found in 

 a minute pebble at 554-573 feet. The biotite plates vary in 

 pleochroism from rich chestnut brown to nearly colorless. 

 They are accompanied by a great number of opacite pseudo- 

 morphs of elongate lath-like form which are seen by transitions 

 to the unaltered biotite to be resorption products of former 

 biotites. These and some iron ore grains and slender needles 

 of apatite are cemented by a colorless substance which polar- 

 izes weakly in minute, patchy, ill-defined areas. Its precise 

 nature cannot be determined, but it is thought to be a mixture 

 of felsic minerals, perhaps nephelite, sanidine, and analcite. 

 In other varieties intermediate between the two mentioned, in 

 which both augite and biotite occur, the resorption products 

 are wanting. Considerable variation may be noticed in the 

 augites ; they are sometimes chocolate-brown, sometimes yel- 

 lowish-brown, and sometimes colorless. In regard to the felsic 

 cement, it is sometimes analcite, as mentioned by Dr. Thomas, 

 accompanied by calcite ; in other cases, as at 995 feet, a color- 

 less mineral of low birefringence, which is held to be nephelite, 

 is also present ; consequently, it is not certain that the analcite 

 in such cases, at least, is primary. The texture of these rocks 

 is similar to that of the groundmass of the melilite-basalts men- 

 tioned above, consisting of an interwoven mass of the mafic 

 minerals with a colorless cement. While the minute size of 

 the " rock sections " does not give a fair idea of the textures 

 as to whether they are strongly porphyritic or not, the rather 

 abundant fragments of phenocrystic augite which occur in the 

 powders suggests that they are at least moderately so. 



Classification. — The classification of these rocks in qualita- 

 tive schemes offers some difficulties. Dr. Thomas has struck 

 the most important thing about them in remarking that they 

 are to be considered the effusive equivalents of monchiquites ; 



