Mineralogy and Petrography. 
(General Kotejs, 
MINERALOGY AND PETROGRAPHY, i 
Petrographical News. — An interesting paper on the origin of 
the hornblende schists and granulites of the Lizard, by McMahon,* 
contains some new ideas with regards to these rocks. The author 
thinks that the banded hornblende schists were originally volcanic 
ashes, along the sedimentation planes of which water flowed, dissolving 
substances in some places and depositing them at others. The horn- 
blende crystals in certain places attracted to themselves new horn- 
blende material and thus produced a dark band. The banding of such 
schists is thus supposed to be due to segregation. Their composition is 
essentially hornblende, plagioclase, and malacotite. The granulites are 
plagioclase, mica, quartz rocks containing a few other unimportant constit- 
uents. They are markedly banded with dark and light bands, the great 
differences in the composition of which are accounted for on the supposi- 
tion that the rocks were originally diorites cut by granite veins, and that 
afterwards they were changed as above outlined. Another valuable 
paper upon a kindred subject is that by Callahan^ upon the produc- 
tion of gneiss and schists by the shearing of eruptive rocks. The 
diorites of the Malvern Hills have undergone a structural change along 
shearing zones wnthout changes in their mineralogical composition. 
The hornblende of these rocks is fractured. It breaks into little grains, 
and diminishes in quantity, until in the zone of greatest shearing it is 
entirely replaced by epidote, chlorite and biotite. The plagioclase 
also decreases as the schistisity becomes more marked, and gives rise to 
muscovite. At the same time secondary quartz and new feldspar are 
generated. In some instances the final stage of the alteration is a rock 
composed of quartz, some feldspar and a little biotite. The alteration 
of the biotite and chlorite into muscovite, the production of garnets 
and zoisite, probably from chlorite, the change of almenite into sphene, 
and the formation of actinolite, hematite and calcite are discussed, and 
the description of many thin sections of rocks are given. It is shown 
that infiltration occurs along the shearing zones, and takes part in the 
» Edited by Dr. W. S. Bayley, Colby University, Waterville, Me. 
* Quart. Jour. Geol. Soc, Aug., 1889, p. 519. 
' Quart. Jour. Geol. Soc, Aug., 1889, p. 475- 
