534 THE AMERICAN NATURALIST. [Vou. XXXII. 
the porphyry and the felsite are called aporhyolites by the author, 
though he does not attempt to prove that their present features are 
due to the devitrification of an ancient glass. The granites comprise 
four types, distinguished as dioritic and hornblendic granites, gran- 
itite and hornblendic diorite. . Nearly all these rocks had already 
been described by earlier writers. White adds a few points of 
interest concerning them. 
The Eruptive Rocks of Mexico. — Harrington ' gives a résumé of 
an article by Ordoñez, in which are described briefly the eruptive 
rocks of Mexico. ‘The precretaceous eruptives are principally gran- 
ites, associated with sedimentary rocks, and sometimes with younger 
rhyolites and andesites. With the cretaceous age began a great 
series of eruptions whose products were granites, granulites, syenites, 
diorites and diabases, and the “ greenstones ” characteristic of the 
mining districts. Among these latter are andesites, green dacites, 
trachytes, rhyolites, labradorites, and basalts. The rhyolites of 
Chichindaro, of San Ildefonso, of Tula, of Hidalgo, and a few other 
places are sphemlitic. Some of the modern volcanoes erupt andesites, 
and others trachytes. Many of the trachytes contain olivine, and 
occasionally these rocks grade into typical basalts. Labradorites are 
also common lavas. They differ from basalts in containing but little, 
if any, olivine. 
The Gneisses of Anglesey, England.?— The gneissic series of 
Anglesey, England, comprises plutonic rocks that have suffered 
crushing and shearing subsequent to their consolidation. The 
banded gneisses were formed from a complex of diorite and felsite, 
or from felsite whose secondary structure has been accentuated by 
the infiltration of dark-colored minerals along the cleavage planes. 
The normal gneisses of the district were formed from granite, diorite, 
or felsite. The hälleflinta, so frequently mentioned in the literature 
of the district, is a partially altered felsite. 
Syenite Porphyries of the Lake Champlain District.— In the 
pre-Potsdam area of Clinton County, N. Y., Cushing’ finds a series 
of dikes, composed of a basic rock which is classed as syenite- 
porphyry. This rock consists of a microperthitic intergrowth of albite 
and orthoclase, biotite, magnetite, hematite, hornblende, quartz, albite, 
1 Journ. of Geol., vol. v, p. 466 
2 Quar. Journ. of Geol. Soc., vol. liii, p. 349, 1897. 
3 Bull. of Geol. Soc. of Amer., vol. ix, p. 239- 
