NORTH AMERICAN GEOLOGY. 223 



oped as augite- trachytes, some of which are very rich in augite. The 

 basalts are in part plagioclastic ; another i^art consist of analcite (no- 

 seau) basalts, originally composed of nosean augite and olivine, with 

 the former changed to analcite — generally occurring in dikes, often 

 crowded in great numbers, and apparently older than trachytes.* 



99. Kewberry quotes the results of examination by Iddings of some 

 of the igneous rocks from the Lower Silurian and Cretaceous of the 

 Belt Mountains, and consisting of typical augite-andesites, true tra- 

 chytes and rhyolites.t 



100. Becker, in a paper on the Washoe Rocks, discusses Hague and 

 Iddings' criticism on the petrography in his monograph on the Comstock 

 lode. An account is given of a re-examination in the field, the result 

 of which substantially corroborates his previous conclusions that there 

 were two separate eruptions of porphyritic, pyroxenic, and plagioclase 

 rocks, presenting sufficient differences to be separated into diabase and 

 andesite. Additional reasons were also found for maintaining the exist- 

 ence of diabase, and for dividing the pyroxene andesite into two distinct 

 outflows separated by a long interval of time, contrary to Hague and 

 Iddings' opinion that the rocks constitute substantially a single Tertiary 

 eruption. The structural features and petrography of the rocis in 

 question are discussed in detail, and while somewhat slight corrections 

 axe made to his previous statements, he finds Hague and Iddings' 

 hypothesis of progressive crystallization inapplicable in explanation of 

 the differentiation of the several members of the Comstock district.| 



101. In a paper on columnar structure in the Mesozoic igneous rocks 

 of New Jersey, Iddings describes -the petrography of the diabase near 

 Orange, New Jersey. It is found that it differs in some respects from 

 most of the similar igneous rocks in that part of the country. " Gener- 

 ally the microstructure of these rocks is holocrystalline, formed of lath- 

 shaped, basic feldspar, irregular crystals and grains of augite, grains of 

 iron oxide, and considerable green serpentine or chlorite, which is dis- 

 seminated through the mass, and is evidently the alteration product of a 

 fourth primary constituent." The rock from the quarry described " is 

 not holocrystalline, but contains a variable amount of glass base, which 

 is more or less globulitic, with augite microlites having opaque grains 

 attached, besides larger aggregations of magnetite grains. There is a 

 comparatively small amount of serpentine in patches, the larger of 

 which still contain fragments of olivine at their centers, the former 

 mineral from which the serpentine has been derived. In some places 

 the glass base has been colored green, though still isotropic, while in 

 others it has deen devitrifled through decomposing agents. The rock 

 with the least glass and coarsest grain of crystallization is from the 

 large columns" near the base of the trap sheet, while that near the same 



* Tenth Census : Report on Mining Industries, pp. 719-737. 

 t New York Acad. Sei., Trans., vol. 5, pp. 247-270. 

 t California Acad. Sci., Bulletin No. 6. 



