578 The American Naturalist. [July, 
Chalcedony Concretions in Obsidians from Colorado.— 
Patton? describes the occurrence of large opal and chalcedony concre- 
tions or geode-like bodies in beds of a decomposed obsidian on Ute 
Creek in Hinsdale Co., Colorado. The concretions are most common 
in the upper scoriaceous portions of the flows. Similar concretions 
were also found in a rhyolite at Specimen Mountain. The concretions 
are composed of radial fibres of chaleedony. The flowage lines that 
are common to the rock pass uninterruptedly through them, and in 
them are trichites exactly like those in the body of the rock. The con- 
eretions are regarded as secondary in origin—and as due to the perco- 
lation of silica-bearing waters through the rock. The same author 
publishes some photographs of erosion forms produced by the weather- 
ing of the volcanic conglomerates in the San Juan Mountains. 
Basic Dykes near Lake Memphremagog.—<According to 
Marsters‘ the Chazy limestones of Lake Memphremagog are cut by 
granite, olivine, diabase and lamprophyre dykes. The latter comprise 
dark rocks containing phenocrysts of augite, hornblende or olivine. 
The olivine, when it occurs, is always situated in the central portions 
of the dykes. Sometimes its crystals are one and half inches in diame- 
ter. Petrographically these rocks are augite camptonites, fourchites 
and monchiquites. The augite camptonite contains both augite and 
hornblende in two generations and in varying quantities. Only two 
fourchite dykes were observed. Their material presents no unusual 
features. The paper is interesting as bringing to our knowledge 
another area in which these peculiar and interesting dyke rocks occur. 
The Origin of the Maryland Granites.—The last article writ- 
ten by the late Dr. Williams’ is an introduction to Keyes article on 
Maryland granites. In this paper the author explains the criteria by 
which ancient plutonic rocks may be recognized in highly metamor- 
phosed terranes, and applies the principles thus established to prove 
the eruptive nature of many of the Maryland granites. The pegma- 
tites of the Piedmont plateau were tested by the same criteria, with the 
result that these too are pronounced to be eruptive. Many handsome 
plates embellish this portion of the papet. In the main portion of the 
article Keyes describes the petrographical features of the different types 
of granite, giving special attention to the original allanite and epidote 
found in them. There is little that is new in the paper, most of its 
3 Proc. Colo. Scient. Soc , Nov. fl 1895. 
t Amer. Geol., July, 1895, p. 
515th Amn, Rep. U.S. G. S., D 653. 
