Prof. T. G. Bonney—Effects of Pressure on Limestones. 485 
regarded as indicative of the same disturbing cause.’ Not seldom there 
seems to be a tendency to orientation in these lamelle, since they 
either lie in or make angles of about 20° with the rude planes 
of incipient cleavage in the rock; these, of course, being roughly 
perpendicular to the direction of pressure. 
The well-known crystalline limestone of Tiree is an interesting 
example of the same process. This rock was for some time a puzzle 
to me, for the large size of the sahlite grains, which were such as 
might be expected from the geological position of the rock, seemed 
to be incongruous with the apparently compact calcareous matrix. 
One would have expected the latter to resemble the crystalline 
calcite in one of the malacolite limestones in the “Granville series” 
of Canada. But an examination of a set of slides prepared from 
specimens of the Tiree rock has removed the difficulty. The 
brecciated or conglomeratic structure above described is at once 
revealed by the microscope. Grains of calcite, of various sizes, in 
some cases almost as large as those of the sahlite, are scattered about 
in a finely granular matrix of calcite, which gives the usual indica- 
tions of having suffered from pressure, although subsequently it has 
been completely reconsolidated. On closer examination, we find 
here and there a grain of calcite, either occupying an inlet in one of 
the sahlite grains or sheltered between two adjacent grains of the 
same, which has evidently formed part of a larger grain, and which 
indicates that the calcite matrix was once in a coarsely crystalline 
condition and that its grains corresponded in magnitude with that 
of the sahlite. In one case that I have examined the accidental 
proximity of some sahlite grains has so protected the calcite that 
two or three crystals of it remain unbroken, and we can, as it were, 
study a fragment of the original rock. Jn one instance, adjacent to 
an unbroken grain of calcite, are some of much smaller size, which, 
in their relation to the matrix and their disposition, suggest that an 
original grain, thus imperfectly protected, has been broken up but 
not crushed. The matrix of the slide consists of calcite granules, in 
which a faint streaking is seen, suggesting the action of pressure. It is 
remarkable that the sahlite grains do not appear in my specimens to be 
broken or even distorted. Probably they have been saved by the easy 
cleavage and brittleness of the calcite, which was reduced to a powder 
by the pressure, and again consolidated as the latter diminished. 
Miss C. A. Raisin has recently shown me a specimen, collected by 
herself, in which the effect of pressure on calcite is well illustrated. 
It is a brecciated volcanic rock (? andesite) from Porth Oer, north of 
Aberdaron, the fissures in which have been filled up by calcite with 
a little quartz. In consequence of subsequent pressure the quartz is 
displaced and shows strain-shadows, but the calcite exhibits various 
stages of crushing, as described above, most of it being reduced 
to granules, and these sometimes are slightly ‘streaky’ in their 
arrangement. Here then the history of the rock is complete and 
there can be no doubt as to the cause of the structure. 
Hence the observations—and they have been rather extensive— 
1 Rutley, Rock-forming Minerals, s.v. Calcite. 
