612 The American Naturalist. [July, 
gabbros and diorites of the region, which appear as though basic 
separations from the same magma as that yielding the granite. The 
most abundant rock is a muscovite granite. Next in importance is a 
uralite-porphyrite, whose uralitic phenocrysts are complete pseudo- 
morphs of augite. All the constituents of the rock show much alter- 
ation. The plagioclase is changed to epidote and zoisite, and between 
the secondary products of this mineral are newly formed plagioclase 
and hornblende, and in addition there are frequently accumulations of 
biotite, whose form leads to the supposition that they are pseudomorphs 
after olivine. In its original condition this rock was probably a basalt. 
A plagioclase-porphyrite, an amygdaloid and glassy rocks with the 
composition of an acid basalt also occur in the region. Tufas accom- 
panied the outflow of basalt, but in this as in the other rocks described 
the character of the original substance has been greatly obscured by 
alteration. In discussing the cause of the chemical changes that have 
been effected, the author ascribes the most powerful action to water in 
connection with pressure. Many of the rocks show evidences of 
dynamo-metamorphism. A schistosity has been superinduced in nearly 
all of the types, but the crushing and breaking of grains that are such 
striking phenomena in most instances of this kind, are here absent. 
The pressure exerted its influence principally in increasing the solvent 
power of the water. Very little change in the chemical composition 
of the rocks has resulted from the alteration, in spite of the fact that 
their mineralogical composition has been totally changed. 
Petrographical Notes.—The breccias and porphyries of Pilot 
Knob, Mo., have repeatedly been stated to be metamorphised frag- 
mentals. Haworth? has examined their relations to other rocks and 
has carefully studied their thin sections with the result that they are 
pronounced by him true eruptives, the latter, quartz-porphyries, 
exhibiting flowage structure, and other evidences of having once been 
liquid, and the former, porphyry breccias, with fragments of porphyry 
cemented by a groundmass that was once a fluid volcanic lava.—— 
Cordierite-bearing chiastolite schists are briefly mentioned by Klemm* 
as forming part of the contact belt of the Lausitz granite at Dubring, 
and dykes of hornblende-porphyrite as cutting the granite at this 
place and at Schmerlitz, in Saxony——lIn a brief communication 
Kemp’ speaks of the existence of several dykes of a very much altered 
1Bull. No. 5, Geol. Surv. of Mo., p. 5. 
*Zeits. d. d. Geol. Ges., xliii, 1891, p. 526. 
3Amer. Jour. Sci., Nov., 1891, p. 410. 
