^5^ The Americari Naturalist. [March, 
pyroxene variety passes over into the typical hornblende- 
andesite through stages in which hornblende and pyroxene 
are both present, the amount of the one increasing with dim- 
inution in the amount of the other. The hornblende is often 
surrounded by an opacitic rim in which are frequently numer- 
ous httle crystals of augite. Tridymite is both an original and 
a secondary constituent in all varieties of the andesites, with the 
exception of the dacites in the extreme eastern portion of the 
area studied, where the silica is in the form of opal and porphy- 
ritic quartzes. The biotite present in many specimens contains 
apatite and rutile inclusions regularly arranged, the former 
with their long axes perpendicular and parallel to the^ axis of the 
mica, and the latter cutting each other at angles of 60*^. The 
author describes the course of a silicification process which 
has taken place in some of the rocks, and also the eutaxitic 
structure noted in many of them.— Of the Andes mountains 
in Colombia four distinct ranges are recognized, viz : the 
Western, Central, Eastern and Coast Cordilleras. The struc- 
ture and the rocks of the Central and Eastern ranges have 
recently been studied by Hettner and Linck.^ In the former 
granite, gneiss, crystalline schists, diabase, dacites, andesites 
and clay slates occur. The crystalline schists and the slates 
are regarded as Archaean. In the Eastern range none of the 
younger rocks were found, except a tuff composed of andesite 
material. Among the older rocks found in this area may 
be mentioned a quartzite and a felsophyre.— In a beauti- 
fully illustrated paper on the rocks between the Province of 
Minas Garaes and S5o Paulo, in Brazil, Machado^ describes the 
gneisses and the sedimentary rocks of the region, nepheline- 
syenite, quartz-augite-diorite and olivine-diabase. The last 
mentioned rock occurs i^n dykeform and presents no unusual fea- 
tures. The diorite forms a stock in the gneiss. It contains in ad- 
dition to its essential constituents also hyperstheneandscapolite. 
The most interesting portion of the paper is that devoted to 
the nepheline-syenites. These are pre-Devonian and form 
the plateau of Pogos de Caldas. Three types are distinguished 
— a coarsely granular, a fine grained and a dense and porphy- 
ritic type. They all contain the same components, viz: 
orthoclase, nepheline, aegerine, biotite, sodalite and cancrinite 
(as a decomposition product of nepheline), and grade over into 
one another. Wollastonite, lovenite and epidote also occur 
in some specimens as accessory constituents. Darker fine 
1 Zeits. d. deutsch. geol. Gesell. xl. 1888. p. 205. 
■ Miner, n. Perog. Mitth. ix. p. 318. 
