668 The American Naturalist. [July, 
MINERALOGY AND PETROGRAPHY.! 
Petrographical News.—A contribution to the knowledge of 
the geology of South America has recently been made by Bergt? 
through the study of thin sections of rocks collected in the Sierra 
Nevada and the Sierra de Peryá in the United States of Columbia. 
Bergt has confined himself to a description of a large number of rocks 
that were collected by others, and therefore he has not been able to 
do more than indicate the interesting results which follow from a close 
study of their thin sections. Among the facts of general interest dis- 
covered may be mentioned the formation of secondary epidote from 
augite and olivine in melaphyre, and the production of an epidosite 
theréfrom ; the occurrence of lamellz in uralite of syenite, that have 
became curved through the pressure exerted upon them by a feldspar 
crystal during its growth; the existence of a rim of brown hornblende 
around a grain of uralite, and the occurrence of secondary brown mica 
as a product of the alteration of augite. The writer also discusses the 
nature of uralite, and suggests that the name uralitite be used asa 
comprehensive one for those rocks containing secondary hornblende, 
whose original nature cannot be determined.—Goller3 describes 
in a very careful article a number of lamprophyre dykes cutting 
gneiss and crystalline schists in the Vorspessart in Germany. The 
erystalline schists consist of dioriticand “ augen ’’ gneisses, produced by 
pressure from eruptive rocks, and other gneisses, the history of whose 
origin is unknown. They are cut by dykes of camptonite and ker- 
santite, whose characteristics are minutely described by the author. 
Both contain large quartz and orthöclase grains that are supposed to 
be the remnants of dissolved inclusions, and smaller quartzes that have 
crystallized from the magma. Two varieties of augite were observed ; 
one alters into talc and tremolite through green hornblende, and the 
other into serpentine through the same intermediate product. The 
original quartz is supposed to owe its origin to the physical conditions 
prevailing during the solidification of the rock mass—principally pres- 
sure and the presence of water.— — Still other instances of the occur- 
rence of young rocks with the characteristics of old ones'are de- 
scribed by Reiser* from four localities in the northern Alps. They are 
typical diabases and diabase porphyrites of Eocene age. They con- 
! Edited by Dr. W. S. Bayley, Colby University, Waterville, Me. 
? Min. u. Petrog Mitth., 1889, X., p. 271. 
? Neues. Jahrb. f. Min., etc., B. B. VI., p. 485. 
4 Min. u. Petrog. Mitth., X., 1889 p. 50o. 
