ROCK STRUCTURES, &C. 
151 
lava was in a plastic state ; in the latter these cavities have 
been filled, to a greater or less extent, by various minerals 
deposited as secondary products ( see Agates, p. 54). 
In the next Case of this series are some excellent illustrations 
of the lamination, bedding, jointing, cleavage and faulting of 
rocks — so far at least as such divisional planes can be shown in 
small specimens. A faulted slate , showing a series of fractures 
and miniature dislocations, is especially noteworthy. Finally, 
there are here some beautiful examples of columnar structure, 
both in igneous and sedimentary rocks; and some curious 
concretions and illustrations of other physical structures, 
including the so-called “ cone-in-cone.” 
The following Case contains an exceptionally interesting series 
in illustration of the mechanical deformation of rocks. Here 
we see fossils squeezed, stretched and distorted by pressure ; 
pebbles in conglomerates compressed and elongated, and the 
mineral constituents of other rocks so drawn out that a banded 
or schistose structure is developed. The effects of prolonged earth- 
movement are strikingly shown by a beautiful series of specimens 
from the Alps, in which the rocks, even on a small scale, show 
folding and contortion, amounting in some cases to sharp 
puckering. Nor are equally fine examples wanting from the 
British Isles — as witnessed by many specimens shown in this 
Case from the old rocks of parts of Scotland and Ireland. In 
the north-west Highlands, for instance, vast masses of rock have 
been dragged bodily over one another for many miles, thus 
subjecting the rocks along the gliding surfaces, or thrust-planes, 
to severe shearing stresses, which have profoundly affected their 
physical structure, and even, in places, their mineral composition. 
Here also may be seen some interesting specimens presented by 
Hr. Hans Reusch, from near Bergen, in Norway, in which the 
impressions of Silurian fossils occur in micaceous phyllites or 
schists. 
In the centre of the Petrographical Room, opposite to the Case 
of rock-forming minerals, is a Table Case (D) devoted to 
illustrations of the action of various disintegrating agencies, 
such as wind, water, and ice. It is not easy to illustrate such 
natural operations by museum specimens ; and hence sketches 
and photographs are added to assist in explaining the phenomena 
on a large scale. The first compartment of the case contains 
specimens showing the action of ice. Here are examples of 
rock-surfaces which have been ground, polished and scratched by 
glaciers ; samples of detritus, or moraine-matter, carried by 
moving land-ice and various ice-borne boulders. The larger 
boulders are necessarily placed in some of the lower compartments 
of the neighbouring Cases. 
The effects of mind and mater, as agents of denudation, are 
illustrated to a limited extent ; it is shown, for instance, how 
rocks may be scored and pebbles polished by means of drifting 
sand. And here, too, are some curious examples of the action of 
