166 The American Naturalist [February, 



and like some specimens of this, contains tridymite in its ground mass. 

 The most interesting type studied is hornblende andesite. This also 

 contains tridymite in its ground mass, and also contains parallel 

 growths of biotite and hornblende with OP of the former parallel to 

 co Poo of the latter. The hornblende is much corroded, and new 

 hornblende and feldspar are among the products of its solution. 

 Large numbers of concretions are characteristic of the rock. There 

 are granular aggregates of cordierite, andalusite, sillimanite, feldspar, 

 biotite, pleonast, corundum, rutile, quartz, garnet, zircon and magne- 

 tite, and are sometimes schistose. The author thinks that they were 

 originally inclusions of sillimanite-cordierite gneiss or schist that were 

 altered by contact with the molten mass of the andesite. He strength- 

 ens his supposition by treating cordierite-sillimanite rocks with ande- 

 site material, when he obtains an abundance of pleonast, which is one 



of the most characteristic minerals of the aggregates. The leucito- 



phyres of the Laacher-See region have again been subjected to a very 

 thorough microscopical study. Martin 7 has found' them to consist 

 principally of sanidine, leucite, nepheline, augite, and sometimes bio- 

 tite and melanite phenocrysts in a ground mass of sanidine, nepheline 

 and green augite, together with a little glassy base. He regards them 

 as tertiary in age and separates them into two groups, according to 

 the presence or absence of melanite. The former contain but 48.50- 

 49.25% of SiOj, while in the latter the percentage of this constituent 

 rises to 53-54%. The mineral in the rock from Perlerkopf, thought 

 by Rosenbusch to be perofskite, is melanite. The rocks from Seeberg, 

 called trachyte by Zickel, are phonolites containing green and violet 

 augite and nests of olivine. There appear to be gradations between 

 the phonolites and fasanites. The leucite-tufa of the region is aleuci- 

 tophyre-tuff and the leucitophyre-nepheline tephrites and nephelinites 

 of the Harmebacher Ley are all nephelinites. Some of Selberg are 

 feldspathic basalts and nepheline basalts, the latter with leucite crys- 

 tals altered to zeolites and augites filled with hornblende inclusions. 



At Democrat Hill and Mt. Robinson in the Rosita Hills, Col., 

 are two vents of old solfataras, whose gases have so affected the rhyo- 

 lite surrounding them that two entirely new and unique rocks have 

 resulted. At the former place the original rhyolite is now replaced 

 according to Mr. Cross 8 by a cellular rock composed of alunite and 

 quartz, and sometimes a little kaolin, whose cavities are lined with 

 'Zeit. d. d.geol. Gcsell., xlii, p. 151. 

 *Amer.Jour. Sci., June, 1891, p. 466, 



