Beviews — R. Kuch — Volcanic Bocks of Colomhia. 427 



Dr. Eeiss prefaces the work by a short introduction describing 

 the main features in the relations of the Colombian Andes ; at the 

 northern end of the country these form three distinct ranges, known 

 as the eastern, central, and western Cordilleras. They are separated 

 by the Magdalena and Cauca valley ; to the south these contract 

 till finally the eastern range merges into the central chain. The 

 two lateral Cordilleras are composed of Cretaceous and Jurassic 

 sedimentary rocks, but the central chain is formed of old crystalline 

 schists, gneisses, and Cretaceous eruptives. The difficulty in the 

 study of the relations of these is increased by the fact that the 

 highest peaks are glacier clad. 



Dr. Kiich's description of the rocks is divided into two parts, 

 a general and a special. In the former the rocks are classified 

 petrographically, and in the latter topographically ; in this section 

 a reference map would have been of great assistance. 



The rocks described are nearly all Andesites and Dacites, and the 

 work is mainly of value as giving a detailed study of these two 

 groups, which are represented by very varied series, and a discussion 

 of their relations. The author agrees with Eosenbusch in regarding 

 the Dacites as entitled to rank as a distinct group, equivalent to that 

 of the Andesites, and not as merely a type of the Amphibole or 

 Mica Andesites. The Colombian volcanoes have yielded a very 

 complete series of representatives of the two groups, as the silica 

 percentage ranges from 54 per cent, to 78 per cent., so that all 

 stages from felspar basalts to quartz trachytes are represented. 

 Dr. Kiich objects to Giimbel's classification into the two groups of 

 the basaltic and trachytic types, and to Eosenbusch's classification 

 of the dacites into the holocrystalline, felso-dacite, andesitic- and 

 hyalo-dacite. The arrangement which he proposes divides both 

 andesites and dacites into three groups; in the former the divisions 

 are pyroxene-andesite, amphibole-pyroxene-andesite. and amphibole- 

 andesite ; in the latter are the " biotite-amphibole-dacite, pyroxene- 

 amphibole-dacite, and pyroxene-dacite. These divisions may be 

 useful for the purposes of a petrographical description of cabinet 

 specimens, but they do not seem to be of value in the field, as the 

 author points out that a transition may be traced from an augite- 

 andesite to an amphibole-andesite in a single hand specimen; he thus 

 objects to the view of Lagorio and Gumbel that these two types are 

 sharply separated from one another. 



The pyroxene-andesite is the most important of the andesites and 

 a detailed description is given of the rock and its mineral con- 

 stituents ; among these, olivine and quartz are both present as 

 accessories. Pyroxene is usually represented both by monoclinic and 

 rhombic species, usually augite and hypersthene. The plagioclase 

 ranges from andesine to bytownite, as is shown by the specific 

 gravity of isolated crystals, a method which the author has applied 

 somewhat extensively. 



The amphibole-pyroxene-andesite is intermediate between the 

 other two types; in it a crystal of augite is often enclosed in one 

 of amphibole and vice versa : in the pyroxene andesite the amphibole 

 never encloses the pyroxene ; the author thus concludes that in the 



