254 '^^(^ American Geologist. -^p'"''- ^^^i 
pressure remains constant. Tlic bubbles act like so many buffers. 
Any capillary tube filled with water interrupted by any insoluble gas 
or liquid possessing a lower surface tension than water, will exhibit the 
same phenomenon. w. o. c. 
The Granitic Rocks of the Pike's Peak Quadrangle. By Edward B. 
Mathews. (Jour. GcoL, 8, 214-240.) 
The granites are regarded as of late Algonkian age ; and four types 
are recognized, differing markedly in texture but agreeing closely in 
mineral and chemical composition. They are typical biotite granites, 
in which hornblende rarely occurs ; and a mechanical analysis of the 
coarse Pike's peak type gave : quartz 33.4 per cent ; microcline, 53-3 ; 
biotite and all minerals with specific gravity above 3.0, 10.7; and 
oligoclase, 2.6; total, 100.00. The petrographic descriptions are ac- 
companied by tables showing for the different types the relative abund- 
ance of the sixteen component minerals, and the relative size and de- 
velopment of the three essential constituents. Strangely enough, fluorite, 
which occurs in three of tlie four types and is very marked in the Sum- 
mit type, is not mentioned as a constituent of the Cripple Creek type, 
notwithstanding its prominence as a feature of the Cripple Creek ore 
deposits. Another table gives the chemical composition of all but the 
Cripple Creek type, in four of Hillebrand's complete and careful anal- 
yses, with determinations of seventeen constituents. When the indi- 
vidual analyses and their average are reduced to molecular proportions 
and compared with analyses of other granites, the Pike's Peak gran- 
ites are seen to be exceptionally rich in silica and potassium, as well 
as in fluorine ; and the family likeness of these types suggests their 
origin in a common magma relatively rich in the elements named. Al- 
though this tends to make the relative ages of the types of little mo- 
ment, it may be noted that on page 22;^ the Pike's Peak type is de- 
scribed as clearly the oldest ; while on page 228 this distinction is as- 
signed to the Summit type. The reviewer's observations in this field 
suggest the advisability of regarding the Pike's Peak type as the nor- 
mal granite of the batholith ; the Summit type and the Cripple Creek 
types as representing a once-continuous contact zone of the batholith, 
in which the remnants of the Algonkian cover chiefly occur ; and the 
fine-grained type, which occurs only as dikes in the other types, repre- 
senting residual magma intrusive in shrinkage and other cracks. Ac- 
cording to this view the contact-zone types are the oldest and the fine- 
grained or dike tj^pes the youngest, while the Pike's Peak or normal 
type is intermediate, having solidified after the contact zone by which 
it is covered and before the dikes by which both it and the contact zone 
are intersected. As might be expected, the gneissoid phase is chiefly 
characteristic of the outer or border portions of the massif. 
w. o. c. 
Geology of the Little Belt Mountains, Montana, with notes on the Min- 
eral Deposits of the Xeihart. Barker, }'(>i^o and Qtlier Districts. By 
Walter Harvey Weed, .-iccoinpanied by a Report of the Petro- 
graphy of the Igneous Rocks of the District. By L V. Pirssox. 
