
1002 The American Naturalist, [November, 
MINERALOGY AND PETROGRAPHY.! 
Petrographical News.—The gabbros of the United States, which, 
until a few years since, were scarcely known, have recently been studied 
in typical regions. Prof. Chester? has lately communicated the results 
of his study of the great belt of these rocks crossing the northern part of 
Delaware and running southwesterly until it unites with that investigated 
by Williams in the Baltimore area. The rocks of the Delaware region 
differ from those of the Maryland area in that the former very fre- 
quently contain quartz. The normal rock is a hypersthene-gabbro, 
containing brown hornblende and biotite. This graduates into a more 
acidic phase by an increase in quartz, and at the same time an equally 
noticeable gain in biotite, until it becomes more properly a pyroxene- 
granite than a gabbro. On the other hand, by the increase of brown 
hornblende, regarded as original, and the assumption of a schistose 
structure, the normal gabbro grades into a gabbro-diorite or a horn- 
blende-gneiss. Further, uralitization of the pyroxene gives rise to 
green schistose rocks, identical in nearly all of their characteristics 
with the gabbro-diorites of the Baltimore region. The author describes 
in detail each type found by him, and gives analyses of the feldspars 
of many of them. ‘The plagioclase of the typical diallage-hypersthene 
gabbros is Ab,An,, while that of the more acid biotitic rock is Ab,An. 
Gabbro-diorite is the name given to the schistose rock in which brown 
hornblende predominates over pyroxene. Since the hornblende is 
regarded as original, there would seem to be no sufficient reason for not 
calling the rock a diorite-schist, thus reserving gabbro-diorite for those 
schistose phases of gabbro in which the hornblende is largely second- 
ary. By the loss of nearly all of their pyroxene the gabbro-diorites of 
both classes pass into hornblende-gneisses. In the gabbro-granites, 
derived from the gabbros by an increase in quartz and biotite, there 
are evidences of pressure action in the shattered condition of the quartz 
and feldspar. Norites, described by the author, are aggregates of 
quartz and feldspar, in which are imbedded phenocsyts of hypersthene. 
— Messrs. Campbell and Brown ê add two new varieties to the Triassic 
traps of Virginia, differing from those described from more northern 
localities in that they contain hypersthene. One is a hypersthene- 
1 Edited by Dr. W, S, Bayley, Colby University, Waterville, Me. 
3 Bull. U. S, Geol. Sur., No, 59. 
3 Bull, Geol. Soc, Amer,, Vol, II,, p. 339. 



