24 
REPORTS OF SOCIETIES. 
Jan., 1890. 
The professor showed how, by observation of the similar results of 
the natural processes of denudation and deposition in the geological 
field, tracing back from shale, through sand, to boulder formations, 
the old coast line of continents, with their estuaries, could be mapped 
out with considerable certainty, and how the limits of the shallow 
sea, in which grew the corals of the Headington beds (coralline 
oolite), could be traced by a somewhat similar process of 
reasoning. In Headington Quarry they had the limestone rock 
containing its corals very little injured, apparently in situ on the 
place where they actually grew. At Wheatley, a mile or two 
further on, they had the same formation, but with no corals 
entire, most of them absolutely comminuted, probably by the action 
of the breakers, showing that there was there the seaward edge of the 
ancient reef. Beyond, no trace of coral is found, but clay, the deposit 
of a sea deep enough to be undisturbed by breakers.—December 
17th. The Rev. J. W. B. Bell in the chair. Professor A. H. Green 
delivered a lecture on “ The Way in which Rocks have been Altered, 
Crushed, and Re-made by Pressure.” Briefly noting the facts of the 
deposition, subsequent upheaval, and frequent contortion of the sedi¬ 
mentary rocks, the professor illustrated by specimens, diagrams, and 
lantern slides of microscopic sections, the resemblances and points of 
difference between a slab of slate and of clay-stone from a coal bed. 
The original elements of both were alike—sand and mud bands. The 
main differences were :—In the clay-stone, fracture along the plane of 
bedding; in the slate, increased hardness, folding of strata, fracture 
parallel to the axis of the fold = slaty cleavage. Pressure might have 
produced these effects. Microscopic sections showed uncleaved rock 
to consist of grains of all shapes and sizes, jumbled together without 
order. The cleaved rock consisted of similar particles, all more or less 
lenticular in shape, with their long sides parallel. An experiment 
illustrating this effect was shown, rough lumps of clay being squeezed 
together in a glass mould. The homely illustration of the manufacture 
of puff paste was then used to show the effect of pressure in producing 
flaking, i.e., cleavage. After dwelling on the evidence afforded in the 
great mountain chains of the enormous force that has been brought to 
bear in crushing and grinding the rocks, the lecturer passed on to con¬ 
sider the geological puzzle of the formation of the crystalline schists. 
Comparing granite with gneiss, each composed entirely of crystals of 
quartz felspar and mica, he pointed out that the component crystals of 
the latter showed rude parallelism produced by lenticular forms. 
Noting the evidence, always present in crystalline schists, of tremen¬ 
dous crumpling, he considered the force which had produced this to 
have been likely to have had a good deal to do with making them what 
they were. The said force, with high temperature, was also declared 
able to bring about chemical changes; instanced by the decomposition 
of glass into silicate by water in a closed tube, exposed to pressure and 
heat. Crystals, too, found naturally in rock, could be produced 
artificially by these means. Evidence that crystalline schists have 
been formed by this process (dynamic metamorphosis) has been dis¬ 
covered in the Alps and elsewhere. A gradual passage can be traced 
from sandstone to schist, with evidence of the different stages. Broken 
fragments are seen to be crushed, then flattened and rolled out; then 
the softer ones are ground to powder as in a mill (milenite), and 
forced to flow in winding curves round the harder grains. This last 
produces eye-like appearances in the texture of the rock (German, 
Aupen Structur ). Then even this is finally ground down and 
obliterated. All the while chemical action has been going on, fresh 
minerals forming and arranging themselves in approximately parallel 
beds ; and, as the result of all this, the original sandstone has now 
become a crystalline schist. 
