704 The American Naturalist. — [August, 
which is a coarse granular rock, on its periphery often becomes finer 
grained and porphyritic. Large biotites and hornblendes are scattered 
through its groundmass, which remains fine grained, and the rock 
thus takes on a prophyritie habit. At other times the decrease in the 
size of its constituent grains is accompanied by a decrease in the pro- 
portion of plagioclase and quartz present in the rock and a large 
increase in the orthoclase present, while hornblende disappears com- 
pletely. It is unnecessary to give the petrographical details of the 
author’s paper. It should be mentioned, however, that the feldspars 
are very carefully studied by comparing the differences in their refract- 
ing indices, and many new points are brought out concerning their 
relations to each other. Some of the plagioclases were found to con- 
sist of nuclei of basic plagioclase, enclosing areas of a more acid feld- 
spar identical with an acid peripheral zone. The phenomenon is 
thought to be due to corrosive influences. In addition to the various 
phases of the tonalite mentioned, the author makes a careful study of 
the veins cutting them, and of the slight alterations they have suffered. 
and he refers to the existence of gneiss fragments occasionlly met with 
in their peripheral portions. 
Petrographical News.—McMahon’ cites, as evidence in favor of 
the eruptive character of the Dartmoor granite, and in opposition to 
the view of Ussher that it resulted from the fusion by pressure of pre- 
existing pre-Devonian sedimentaries, the following facts. Its apophy- 
ses cut the surrounding rocks. The metamorphic changes effected in 
the latter are the result of contact action. Finally the other rocks 
with which the granite is associated show no evidence of the great 
pressure, to which they must have been subjected if the granite were 
truly a fused sedimenta 
Associated with the argillites, graywackes and other sedimentary 
rocks of the Keewatin series near Kekaquabie Lake in Northeastern 
Minnesota, Grant? has discovered volcanic fragmentals and amphibole 
schists, the former of which are recognized as diabase tuffs and the 
latter as their recrystallized representatives. 
A quartz bearing leukophyre variety of diabase porphyrite, forms 
intrusive layers in the Carboniferous schists at the Hernitz Mine near 
Saarbrücken in the Pfalz? The rock was regarded by Weiss as a 
melaphyre. 
"Quart. Journ. Geol. Soc., XLIX, p. 385. 
Proc. Somerset Arch. & Nat recs "Soc, Vol. 28, p. 892. 
*Science, XXIII, 1894, p. 1 
*Laspeyres : Corr. Blatt. ge^ Ver. Bonn., 1893, p. 47. 
