1394.] Petrography. 1019 
General Netes. 
PETROGRAPHY: 
Composite Dykes on Arran.— Professor Judd? describes a num- 
ber of * composite" dykes on the Island of Arran, in which the well- 
. known “Arran pitchstone” and a glossy augite-andesite occupy differ- 
ent portions of the same fissure, either rock appearing in the center of 
the dyke, with the other on one or both of its peripheries, or the one 
rock cutting irregularly through the other. The relations of the rocks 
indicate that there was no regular sequence in the intrusion, the pitch- 
stone having been intruded sometimes before, sometimes after the and: - 
site. Each rock contains fragments of the other (in different dykes), 
and the two rocks are always separated by a sharp line of demarkation. 
The andesite is a basic rock containing about 56 per cent of silica, 
while the pitchstone is a pantellerite with 75 per cent of SiO, or an 
augite-enstatite-andesite with 66 per cent of SiO, and 4.13 per cent 
K,O. The andesite is well characterised. It passes into a tholeite 
with intersertal structure, by a decrease in the glassy component, and 
upon further loss of glass it passes into diabase. The pitchstone is 
largely an acid glass, surrounding crystals of quartz, and microlites of 
augite, feldspar, magnetite, ete. The author adds to the list of individ- 
ualized components already known to exist in the rock hyalite and 
tridymite. The latter mineral occurs in plates aggregated into spher- 
ules and globules that surround quartz crystals, and the hyalite forms 
globules scattered here and there through the glass. The author thinks 
that materials of such widely different nature as that existing in these 
dykes could not have been formed by the differentiation of a magma 
after its intrusion into the dyke fissures, but that the differentiation 
must have taken place while the magma was still in its subterranean 
reservoir. 
Analyses of Clays.—Hutchings' quotes a series of analyses of 
carboniferous clays to show that these substances possess the requisite 
composition to become clay slates upon compression. He ascribes the 
small percentages of alkalies shown in most clay analyses to the fact 
‘Edited by Dr. W. S. Bayley, Colby ne Waterville, Maine. 
? Quart. Jour. Geol. Soc., xlix, 1893, p. 5 
* Geol. Magazine, Jan. add Feb., 1894. 
