280 THE AMERICAN NATURALIST. [VoL XXXIII. 
in places by flows of felsite-porphyry, granite-porphyry, syenite-por- 
phyry, hornblende-vogesite, diorites, diabase, and augite-porphyrite. 
The greatest interest of the paper lies in the discussion of the nature 
of the weathering product, laterite, which here, as in other tropical 
lands, constitutes so large.a part of the rock covering. In the Sey- 
chelles this material results from the decomposition of both acid and 
basic rocks, but it is best developed in connection with the granite, 
bowlders of which may consist of the fresh rock in the center and 
laterite on the exterior, with a complete series of gradation forms 
between. The typical laterite is a red, brown, or yellow mass that 
may be dense and hard, clay-like, or sandy and friable under differ- 
ent conditions. Often this substance may be mixed with quartz 
grains or mica scales. In thin section it is sometimes nearly opaque, 
sometimes completely transparent. Everything but the quartz of the 
granites and the ilmenite of basic rocks has been changed to a light- 
colored, scaly aggregate of doubly refracting plates colored in places 
by iron oxides and other compounds. Analyses of this substance from 
_ granite and diorite yield: 60.68% Al.O;, 9.56% Fe.O;, and 29.76% 
H-0 for granite-laterite, and 51.98% Al.O;, 20.95 % Fe.0;,and 27.07% 
H-0 for diorite-laterite. 
Laterite is thus very different from clay ; in composition it is much 
more like hydrargillite. The beauxite of the Vogelsberg and other 
supposed beauxites derived from basalts are of the same nature. In 
all cases the laterite is the residue left by leaching agents in a tropi- 
cal climate. The occurrence of the beauxite at Vogelsberg indicates 
to the author the existence of a warm climate over this place at the 
time the beauxite was formed. 
Isenite and Intermediate Types of Volcanic Rocks. —In the 
Westerwald, in the province of Hesse-Nassau, basalts, trachytes, 
andesites, and phonolites are well developed in many different 
phases, especially in the transition forms that have recently attracted 
so much attention among petrographers. The predominant andesite, 
for instance, is a transition phase between andesite and trachyte; 
some of the other andesites are basaltic in habit, and a few of the 
trachytes are phonolitic. Dannenberg! describes all these types in 
detail, and adds analyses of many of them. The “isenite” from 
Sengelberg, Kramberg, and Himmrich consists of a groundmass 
made up of lath-shaped plagioclases, and small grains of augite and 
of olivine, magnetite, and some secondary substances, and pheno- 
1 Min. u. Petrog. Mitth., Bd. xvii, pp. 301, 421. 
ç 
