512 The American Naturalist. 



[.Inn, 



Costal area into two parts, a Western one underlain by fragmental 

 rocks in which but slight alteration has been effected, and an Eastern 

 one characterized by both sedimentary and eruptive rocks that have 

 been strongly metamorphosed by pressure. In the first area phyllites, 

 including sericite, chlorite and ottrelite schists, sandstones made up of 

 undoubtedly clastic grains, and a few crystalline limestones occur. In 

 the Eastern area the piedmont rocks are gneisses, some of which are 

 evidently eruptive and others probably sedimentary ; quartzites and 

 quartzite-schists, in which all evidences of fragmental origin have dis- 

 appeared, coarse crystalline marbles containing phlogopite, tremolite, 

 scapolite, etc., and acid and basic eruptives. Each of these classes, 

 except those belonging to the eruptive division, is briefly character- 

 ized, and pictures 1 of the sandstone of the Western area and of the 

 quartzite of the Eastern area are given for comparison. 



The Diorite of the Andes, first mentioned by Stelzner as a 

 chanicttM-i.-tie rock >>( this mountain range, has been closely studied by 

 Moricke 1 in the occurrence at St. Cristobal near Santiago in Chile. It 

 is closely associated with andesites, both hornblendic and augitic, with 

 propylite and tufas, in such a way that the author is led to look upon 

 them all as facies of the same rock mass, the diorite representing the 

 deep-seated phases and the andesites the surface flows. The horn- 

 blende, augite and hypersthene-augite, andesites and the diorites have 

 the usual characteristics of these rocks. With respect to structure an 

 intermediate phase occurs in the propylite, which consists of pheno- 

 crysts of plagioclase and green hornblende in a ground mass of plag- 

 ioclase, chlorite and epidote. The propylite is much altered, while the 

 other rocks are fresh. The geological relations of the different rock 

 types correspond with the conclusions outlined as above. The ' stocks ' 

 of diorite, often forming the peaks of the range, are the denuded cones 

 of old massifs. 



The Porphyry of Monte Doja.— Pelikan s has re-examined the 

 rock discovered by Suess at Monte Doja, in the Adamello group of 

 mountains, and described by him as a reddish brown porphyry. The 

 structure is porphyritic and the groundmass is dark brownish red in 

 color. Under the microscope the thin section shows a colorless 

 base containing yellowish-brown wisps of biotite, thin prisms and fine 



l C. R. Keyes, lb. p. 321. 



2 Min. u. Petrog. Mitth. xii, p. 143. 



3 Min. u. Petrog. Mitth., xii, p. 156. 



