484 Prof. T. G. Bonney—LEfects of Pressure on Limestones. 
In the Alps, as is well known, large masses of limestone occur 
which are indubitably of Mesozoic age. Sometimes these have been 
exposed to great pressure; sometimes they appear to have been 
uplifted with comparatively little disturbance. In either case there 
does not appear to be a very marked difference in their crystalline 
condition. A Mesozoic Alpine limestone may be described in 
general terms by saying that it resembles, often closely, in texture 
and sometimes even in colour, one of the Carboniferous limestones 
of Britain—that is to say, it varies from black to grey—though the 
peculiar light grey of the English Carbonifeous limestone is, so far 
as I know, most unusual. Not seldom the rock is yellowish in colour, 
like some of our Mesozoic limestones, but with a different texture ; 
occasionally it is reddish. Even where the calcareous constituent 
exhibits a crystalline character, it is obvious that, if this were 
removed by the action of an acid, the residue would be, roughly 
speaking, mud, silt, or sand which, as a rule, has not undergone 
more alteration than would be observed in the above-named British 
rocks. If, however, we examine a limestone which both is in- 
dubitably associated with a series of crystalline schists and has been 
selected from a comparatively undisturbed region, we find that 
whether coarse or fine (in the latter case of course the statement 
may be less precise) the constituents throughout are in a crystalline 
condition. If quartz-grains are present, they bear no resemblance 
to those in a sandstone, but have evidently assumed their present 
outline in siti.’ If mica, it is in like way evidently authigenous. 
So also with the malacolite, sahlite, or other mineral constituents. 
In short, it is evident that, whether the rock were originally an 
organic aggregate, a chemical precipitate, or a detrital accumulation, 
the constituents have undergone a molecular rearrangement, which 
is in all cases considerable and is often complete. 
If now we turn from the examination of such specimens to one 
of the abnormal-looking limestones, collected from a crystalline 
group which has been exposed to severe pressure, we find that its 
constituents, as before, are in a crystalline condition, but that the 
structure of the rock is different. Instead of finding a considerable 
uniformity in the size of the calcite grains, we observe a marked 
diversity. In a ground-mass of granules are scattered grains 
variable in size and number, subangular or slightly irregular in 
outline, and sometimes exhibiting a slightly linear arrangement. If 
an outline sketch were made of the slide, it might be supposed to 
represent a subangular breccia or conglomerate—except that the 
outline of the larger grains is slightly irregular or ‘ragged.’ 
Further, many of these grains exhibit the twin lamellz, which, 
as is well known, can be artificially produced in a calcite crystal by 
pressure, and which, if they occur in rocks, are now commonly 
‘dolomites’ generally have a more crystalline aspect, and, among the crystalline 
schists, do not usually show mineral cleavage so readily as those which only contain 
calcite ; they also exhibit the usual differences to which I called attention in 1879 
(Q.J.G.8. vol. xxxy. p. 167). 
1 Of course they may have had a clastic nucleus, but of this their present outline 
shows no trace. 
