Prof. T. G. Bonney—Effects of Pressure on Limestones. 483 
partially filled up. (See Pl. XIV. Fig. 3.) The last act to which we 
have to call attention was the filling up of the cavities remaining 
“empty after final consolidation with chalcedony and carbonates. 
We may summarize the history of the rock so far as it is recorded 
in microscopic structure as follows :— 
1. Development of granular aggregates of a felspar allied to anorthite under 
plutonic conditions. 
. Addition of felspar substance to the external portions of the granular aggre- 
gates, and the consequent production of crystalline form. 
. Development of lath-shaped felspars. 4. Separation of augite. 
. Formation of vesicles owing to the separation of gas from the magma. 
. Partial or complete fillimg up of some of these vesicles with interstitial matter. 
. Consolidation of the interstitial matter. [carbonates. 
. Filling up of the vesicles remaining empty after final consolidation with 
EXPLANATION OF PLATE XIV. 
Augite is indicated by dots, interstitial matter by lines. The felspar is left clear. 
'Magnetite is black. 
Fie. 1. Vesicle filled with interstitial matter. 
» 2 45, ~ filled with carbonates. 
» 98 4, partly filled with interstitial matter and partly with carbonates. 
CIO Oreo bo 
I].—Tas Errrcts or Pressure ON ORYSTALLINE LimestTonss.! 
By Prof. T. G. Bonney, D.Sc., LL.D., F.R.S., F.G.S. 
far who have carefully studied the crystalline schists cannot 
fail to have noticed that a community of structure—a sort of 
family likeness—prevails throughout any one group of rocks, while 
those which occur apparently at different horizons exhibit dissimilar 
structures. Thus the marbles associated with any group of 
crystalline schists are coarse or fine in grain, according to the 
structure of the latter. But to this rule exceptions appear, at first 
sight, not infrequent. For instance, in the Alps, we find not un- 
commonly, in that group of schists which seems to occupy the 
highest position, marbles which present an abnormally compact 
aspect. On closer examination they prove indeed to be crystalline 
in structure, but the crystals seem so small, the general structure 
so compact, that until we find them graduating into typical mica 
or other schists, we can hardly feel satisfied we are not being duped 
by infolded limestones of Mesozoic or Paleozoic age. When, 
however, we study the microscopic structure of such crystalline 
limestones, the abnormality proves to be more apparent than real. 
This compact structure is due, not to the absence of crystallization, 
but to the destruction by pressure of an original crystalline 
structure: for we find on examination that the rock, once perhaps 
coarsely crystalline, has been crushed and again consolidated, so 
that it has now assumed a comparatively compact appearance. 
A proof of the above statement may be afforded by a brief 
description of the structures which are exhibited by a series of 
limestones * which J have from time to time collected and examined. 
1 Read before Section C, Brit. Assoc. Newcastle-upon-Tyne Meeting, Sept. 1889. 
2 Many of the Alpine limestones, both in the Mesozoic and in the Crystalline series, 
are more or less dolomitic, but I have not thought it necessary to distinguish these. 
The statements apply to both, except that, among the ordinary sedimentaries, the 
