1888.] Dr Stecher on Contact- Phenomena. 171 
other conclusions. It is true he has also stated that the twin- 
ning of the plagioclase is nothing but a phenomenon produced 
after solidification. But he suggested that the crystals had been 
twinned by a pressure which he interprets as resulting from the 
weight of the rock masses superposed upon the volcanic sheets. 
Assuming such a pressure, I should be unable to explain why the 
above-described phenomena should occur in so striking a manner 
only round an enveloped fragment. 
Besides these chief results, I may state some other interesting data. 
Dr Sorby* has stated, concerning the rock of the Salisbury Crags, 
that, at the j unction of the diabase and sandstone, the fluid-cavities 
in the quartz grains of the sedimentary rock have been emptied 
by contact with the igneous magma. In contradiction to this, I 
succeeded in finding fluid-cavities containing vibrating bubbles in 
the quartz grains, both near the immediate junction and in frag- 
ments totally enveloped by the volcanic mass. Nevertheless, the 
statements of Dr Sorby may be correct, since the specimen he 
examined may have been taken from a point of the volcanic 
mass where the magma altered the associated rocks to a greater 
degree. 
The rocks associated with the olivine-diabases are often black 
Carboniferous shales. Where these shales have come in contact 
with the volcanic rocks, the carbonaceous substance has been driven 
back for a minute distance (at the utmost 2V inch). Thus the shale 
exhibits at the junction a small white border, followed by a zone 
which is darker from having absorbed a great part of what has 
been driven back frchn the immediate junction. The olivine-diabase 
of the Salisbury Crags proves to be rich in microscopic crystals of 
analcime, which, between crossed nicols, exhibit the well-known 
anomalous phenomena. I tried the following experiment : — A slice 
was prepared without application of heat. In this slice the anal- 
cime shows the same beautiful polarisation of light, as when prepared 
by the usual method. This result disproves the statements of 
Eohrbach.t 
Finally, I will cite the result of an analysis of the “ white trap ” 
from Newhalls, near Queensferry : — 
* Address, Quart. Jour., xxx. 
t Tsclitrmak' s Mineral, u. petrogr. Mitth., 1885, vol. vii. p. 32. 
