702 The American Naturalist. [August, 
PETROGRAPY. 
The Ejected Blocks of Monte Somma.—Johnston Lavis’ 
has begun a thorough study of the ejected blocks of Monte Somma, 
with especial reference to their petrography and the nature of the 
metamorphic changes that have been produced in them by the lavas 
by which they were enclosed. The druse minerals of the blocks have 
long been known, but their nature as rocks has been left uninvestigated. 
The author proposes to study in detail about 700 specimens of the 
blocks, including many varieties. He begins by describing some 30 
that were originally stratified Cretaceous limestones containing carbon- 
aceous material The first stage in their alteration seems to be the con- 
version of bituminous substance into graphite, and the crystallization 
of the rock into marble. The crystallization has not destroyed the 
original bedding bands, nor the most delicate structures exhibited by 
them, hence it is assumed that fusion or softening of the rock did not 
accompany the crystallization processes. A few olivines were formed 
at this tisme, and these consequently are the first products of the 
metamorphosing agency. They appear principally as inclusions in the 
calcite. In the next stage of alteration the graphite disappears, and 
a saccharoidal marble results. This contains more or less colorless 
olivine, and passes rapidly into a mass of olivine, colorless pyroxene, 
wollastonite and biotite, where impurities were present in the original - 
rock. In the earlier stages of metamorphism the calcite and the sili- 
cate minerals will exist in different bands, but in later stages silicates 
and calcite intermingle, and finally a purely silicate rock results. 
The order in which new minerals seem to develope is thought to be the 
following ; olivine, periclase, humite, spinel, mica, fluorite, galena, 
pyrite, wollastonite, garnet, vesuvianite, nepheline, sodalite, feldspar, 
secondary calcite, tremolite, brucite. The article is illustrated by three 
lithographic plates. It will repay close study by students of contact 
action, as we have recorded in the blocks the effects of the action of a 
magma upon a limestone, in all its stages. 
Phonolites from the Black Hills.— The sanidine-trachyte 
described by Caswell from Bear Lodge in the Black Hills, has been 
‘Edited by Dr. W. S. Bayley, Colby University, Waterville, Me. 
"Tarns. Edin. Geol. Soc., VI, 1893, p. 314. 
3U. S. Geog. & Geol. Survey of Rocky Mts. 1880. Cap. VII, p. 471. 
