Mineralogy and Petrography. 527 
MINERALOGY AND PETROGRAPHY:.! 
PETROGRAPHICAL News.—In a preliminary notice of the rocks 
occurring in the neighborhood of Ilchester, Howard county, Md., 
Mr. Hobbs? describes some interesting features of the eruptive 
masses of the region. The oldest rock within the area studied is a 
hypersthene-gabbro with its associated alteration products.? In the 
gabbro diorite, originating by dynamic metamorphism from the 
massive hypersthene-gabbro, ilmenite and rutile are found to be so 
associated with sphene as to lead the author to regard the rutile as 
an intermediate product in the alteration of ilmenite to sphene. 
The end product in the alteration of the gabbro is a typical horn- 
blende-gneiss, in which peripheral granulation of the original feld- 
Spar can be detected. A quartz-mica-diorite, in which the quartz 
is in porphyritic crystals, contains about equal proportions of 
orthoclase and plagioclase. The most common form of granite, 
cutting the gabbros and allied rocks, is a coarse pegmatite in which 
microcline crystals a foot in diameter are sometimes observed. 
varieties. The former consists of a glassy type, in which certain 
dark particles group themselves so as to produce patches of a ` 
dark color on a background of light-colored glass, without the 
production of a true globulitic or spherulitic structure. In the 
radiolitic variety devitrification products are divergently arranged 
around a centre.—The nodules of the coarse granite at Ghistorrai, 
consisting of triclinic soda-feldspar and biotite, in 
centric layers. The biotite of the nodule is older than the feld 
Bayley, Colby University, Waterville, Me. 
s Johns Hopkins Univ. Oire: No, 65, April, 1888, p. 68. 
; Ame p. 1049. 
Min. u. Pet $ 
. rog. Mitth., 1887, ix., p. 61. 
* Bull. d. 1. Soe. Franç d. Min., X. ` 57. 
