478 PETROLOGICAL ABSTRACTS AND REVIEWS 
on an average about 84.7 per cent when subjected for 30 days to a 
gradually increasing pressure. When left two years in the apparatus, 
one cylinder had even gained in strength. A marble cylinder deformed 
slowly for 64 days had about double the crushing strength of one equally 
deformed in ro minutes. 
However, deformation of rocks without fracture in nature is not 
believed by all to mean plastic deformation. Grubenmann, Becke, and 
Van Hise have emphasized the importance of mineral elongation normal 
to the pressure by means of solution on maximum pressure surfaces and 
redeposition on surfaces of minimum pressure through the agency of 
water. Opinions are also divided on the relation between rock deforma- 
tion and the deformation of their constituent minerals. A. Heim (1908) 
believes that rock deformation and mineral deformation are distinct. 
A rock may be deformed without fracture as a whole, whereas its 
individual minerals may in part be fractured and others deformed as 
plastic objects. T. Lehmann (1889) regards deformation which is 
accomplished without rock fracture as a whole but by the fracture of 
individual minerals as “bruchlos,’’ deformation without fracture. 
Weinschenk disbelieves in plastic deformation in general, but admits 
its possibility in slates and marbles, and claims they always present evi- 
dence of fracture. He asserts that in many cases schistosity was devel- 
oped by magmas crystallizing under differential pressure. Steinmann 
(1907) is opposed to the latent plasticity view of A. Heim, but admits 
that thickening and thinning of rock strata has been important. C. 
Schmidt (1908) is not opposed to the view that plasticity is possible at 
sufficient depth but believes that Heim has not allowed for sufficient 
depth in his assumptions. C. R. Van Hise, Becke, U. Grubenmann, 
have emphasized the action of water as an agent in causing deformation 
without fracture, but do not deny that plastic deformation may be 
possible. E. STEITMANN 
Pirsson, L. V., and Rice, W. N. ‘Geology of Tripyramid 
Mountain,’ Am. Jour. Sct., ath ser., XXXI (April, 1911), 
269-91. Figs. 6. 
Prrsson, L. V. ‘“Petrography of Tripyramid Mountain,” Am. 
Jour. Sci., 4th ser., XX XI (May, ror1), 405-31. Fig. 2, and 
analyses. 
The first paper describes the geology of Tripyramid Mountain, N.H., 
and discusses the probable origin of the mountain. In the second the 
several rock types entering into the composition of the Tripyramid 
