368 PKOr. T. G. BONNET ON THE GABNET-ACTINOLITE [Aug. 1 898, 



(8) Calcite (or a closely-allied carbonate) is present in some 

 slices ; occasionally it forms fair-sized grains, sometimes it is inter- 

 stitial in a hornblende ' gridiron ' or is moiilded on an idiomorphic 

 felspar-crystal. 



(9) Entile, zircon, and one or two other accessory minerals 

 occur, but are far from common, and call for no special notice.^ 



lY. Infeeences as to Mineral and othee Changes. 



The microscopic studies summarized above fully confirm the 

 inferences suggested by work in the field, that we are dealing with 

 a group of rocks which originally varied from ordinary diorite to 

 hornblende-biotite granites, not rich in quartz, in which garnets 

 are present to a variable extent, and are more common in the latter 

 rocks. In these they also attain a larger size, and are occasionally 

 associated with a little stanrolite. The different members seem to 

 pass, though not seldom rapidly, one into another, and alternate, 

 sometimes on a large, sometimes on a small scale ; in short, they 

 often mimic true stratification. But whatever may have been the 

 origin of the group, after its members became holocrystalline, 

 pressure came into play, and crushed them more or less, the extent 

 depending on their composition and on local circumstances. The 

 garnets generally did not suffer severely : the quartz, when present, 

 was more or less broken up, the felspar was generally much crushed • 

 afterwards it was in some cases converted into paragonite (silica 

 being set free), though a fragment now and then survived, but in 

 others (and more commonly) it went back to a water-clear felspar, 

 not always quite identical in chemical composition, for I regard the 

 dust as representing an unused aluminous silicate. Possibly the 

 difference mentioned may have depended upon whether the water 

 in the rock percolated or was almost stagnant. 



The original hornblende and biotite were also crushed, and when 

 the groundmass was reconstructed, or perhaps at a slightly later date, 

 they were rebuilt, the hornblende with a more or less actinolitic 

 habit. The larger flakes of biotite, which include grains occurring 

 in the groundmass, may have been growing simultaneously with the 

 other mineral, but the smaller flakes associated with it, as described 

 above, must, I think, be later in date. Though not exactly pseudo- 

 morphs, they appear to be determined in position by the hornblende, 



1 Some bed-hke masses of a fairly typical hornblende-schist are found in the 

 Tremola group, occurring, so far as my memory goes, in the flatter part of the 

 valley above the first long series of zigzags. Their relations to the rest of 

 the group are not easily determined, but I suspect them to be intrusive sheets, 

 modified by subsequent pressure. The three specimens which I have examined 

 under the microscope show a more or less foliated structure and the following 

 minerals: hornblende, generally in soniewhat elongated prisms, but hardly 

 actinolites, well cleaved, and a rich green in colour ; biotite in flakes, comparable 

 in size with the other, variable in amount ; secondary felspar ; perhaps a little 

 quartz ; some epidote, iron-oxide (certainly ilmenite in one case), calcite (rather 

 abundant in one specimen), rutile (rather frequent in another), and one or 

 two accessories. These specimens do not suggest, like the others described 

 •bove, a ' rebuilding' of hornblende or biotite on a large scale. 



