
826 The American Naturatst. [September, 
tose, and the mother-rock was a diabase or a diabase-porphyrite. The 
paper concludes with a discussion of fifteen analyses. of the rocks de- 
scribed, Dynamically altered diabase and gabbros (sometimes schis- 
tose) occur also as sheets and dykes cutting altered sediments at St. 
Johnstown and Raphoe, in N. W. Ireland. Among them are epidiorites 
in which, according to Hyland,’ the hornblende still preserves the 
ophitic structure of the original augite. The original feldspar was 
labradorite, but the changes effected in it have yielded a quartz-feldspar- 
epidote mosaic in which the secondary feldspar is oligoclase. e 
schists, granulites, and even some of the massive rocks of the Lizard, 
England, are distinctly banded. To account for the phenomenon it 
has been suggested that it is due either to original sedimentation, or to 
the deformation of eruptive rock-masses, or to the injection of rock 
material along planes of weakness in preéxisting rocks. Since the | 
schists are now known to be eruptive, the first explanation is not avail- 
able. Against the second Somervail® brings the following objections, 
viz.: the symmetry of the banded structure, the frequent transitions 
between adjacent bands, and the uniform banding in large masses of 
the same composition. The injection theory is contradicted by the 
absence of irruptive contacts. He accounts for it on the supposition 
that segregations formed during the cooling of the magmas, 
yielding ‘‘schieren”’ that were afterwards squeezed. A green schis- 
tose rock from near Zermatt, in the Pennine Alps, occurs so associated 
with other schists that Bonney ° is compelled to regard it as a pressure 
schist derived from serpentine. The rock is so. very fissile that it may 
be split into sheets one-eighth of an inch thick. Only two essential 
constituents are observed in it, one an olive-green mineral, occurring 
in small translucent flakes with a cleavage like mica and an extinction - 
parallel to this, and the other a chromite or magnetite. The former 
mineral may possibly be antigonite. Associated with this rock are two 
other schists: one, a green schist, is a soft chloritic rock, composed 
of chlorite, magnetite, and zoisite (?). The composition of the chlo- 
rite is probably near that of chloritoid. The other schist is full of 
talc. The origin of neither of these could be determined. A 
suggestion in explanation of the cause of the transitions sometimes 
seen between crystalline and clastic rocks has recently been offered by 
Prof. Pumpelly," who believes that rock disintegration by weathering 




T Geol. Magazine, 1890, p. 205. 
8 Ib., Nov., 1890, p. 509. 
9 Ib., Dec., 1890, p 
10 Bull. Geol. Soc. pid Vol. i 209. 



