1897,] Petrography. 605 
PETROGRAPHY 
Ancient Volcanic Rocks in Pennsylvania.—Reference has 
already’ been made in these notes to the discovery of ancient acid and 
basic volcanic lavas and tuffs at South Mountain, Pa. Miss Bascom! 
has recently given an exhaustive account of all the types of these rocks, 
which account is beautifully illustrated by reproductions of micro- 
photographs and of colored drawings, and by a large scale geological 
map. The volcanic lavas are partly devitrified rhyolites and partly 
altered basalts. A brief notice of the former was given several years 
ago.‘ The present report adds much of detailed information concern- 
ing them to that already imparted, but nothing of general interest. 
These lavas are pre-Cambrian, and are probably older than the basic 
rocks with which they are associated in the Monterey district. The 
basic lavas were originally diabases, augite-porphyrites and mela- 
phyres. They have suffered extreme alteration in consequence of 
weathering and also as a result of squeezing. Nearly all the rocks are 
schistose, the most highly schistose ones being now practically slates. 
Rocks Associated with the Magnetites near Port Henry, 
N. Y.—The rocks associated with the non-titaniferous magnetites at 
Mineville and near Port Henry, N. Y., are described by Kemp’ as 
gneisses and gabbro. Four varieties of the gneiss are distinguished, of 
which three are acid and one basic. One of the acid gneisses consists 
of quartz and plagioclase exclusively. Another is composed of these 
minerals and a large proportion of micro-perthite, and the third of 
brown hornblende, green augite and rarely hypesthene, in addition to 
the feldspars and quartz. The basic gneiss is a schistose gabbro. It 
grades into the massive gabbro. In some phases hornblende and much 
garnet are present. All the gneisses, as well as the gabbro, are thought 
to be igneous in origin and to be pre-Cambrian in age. The ore de- 
posits are on the contact of the acid and the basic rocks. While their 
method of origin is not certainly known, it is believed by the author 
that the ores may be contact products resulting from the action of the 
intrusive gabbro upon the gneisses intruded by it. 
! Edited by Dr. W. S. Bayley, Colby Wri Waterville, Me. 
? American Naturalist, 1894, pp. 515, 517 and 9 
* Bull. U. S. Geol. Survey, ie 136, gor eg 1896. 
‘American Naturalist, p. 515. 
* Trans. Amer. Inst. Min. Eng., 1897. 
