282 PK0P. T. G. BONNET ON SOME CASES OF THE [May 1 894, 



opacite. The nature of the water-clear mineral is not easy to 

 determine. Some of it is more probably a secondary felspar than 

 quartz. 



On the eastern shore of the Lago Bianco a mass of schistose 

 green rock is intrusive in a rather fine-grained, somewhat crushed- 

 looking gneiss, which also is slightly green in colour. It runs 

 obliquely up the hillside and must be several yards in thickness, 

 but as there is no continuous section the exact figure is not easily 

 ascertained. This rock is coarser in texture and less schist-like 

 than the others. On examination with the microscope it exhibits 

 numerous patches and occasional streaks occupied by an aggregate 

 of minerals, in certain cases apparently an actinolitic hornblende, in 

 others a chlorite, some of the latter being fairly dichroic and changing 

 from a pale tawny yellow to a distinct green (a usual type). The 

 patches (where hornblende is common) are often speckled with 

 opacite. Minute mica is probably present, but is less distinct than 

 in the other cases. 



Rather lenticular ' earthy-looking ' patches (not compound in 

 structure) are numerous; these doubtless represent crushed and 

 decomposed felspars, and in one or two, which retain some clear- 

 ness, the oscillatory twinning of plagioclase can be still recognized. 

 Water-clear grains, often pierced by minute belonites, are fairly 

 common. Most of them present on the whole a closer resemblance 

 to a secondary felspar, though quartz also may be present. There 

 are granules and grains of iron oxide, the larger most like ilmenite. 

 A little calcite is also present. Though a schistose structure is 

 distinct, the rock still retains some likeness to a diabase and would 

 be recognized more readily as a crush-product of a fine-grained 

 dolerite. Its specific gravity is 2-88. 



From these facts it follows that, in certain cases, a rock of igneous 

 origin may be so completely changed by pressure and its indirect 

 consequences as to be readily mistaken for a compact and not very 

 much altered sediment. 1 For this to happen it is, I believe, 

 requisite that the original rock should be glassy or almost in this 

 condition, viz. the segregated minerals, as a rule, should be micro- 



1 The rock from the Llyn Paclarn district which I described in 1879 as a 

 black slate (Quart. Journ. Geol. 8oc. vol. xxxv. p. 312) is a similar case. As 

 rightly determined by the Rev. J. F. Blake (pp. tit. vol. xlix. p. 456), the mass 

 is a dyke. His specimen, which he was good enough to show me, places the 

 igneous origin of the rock beyond doubt, though whether it be a lamprophyre 

 (if that term bears any definite sense) is more open to question. But it 

 differs so widely from my specimen that at first I doubted whether they came 

 from the same mass. I have, however, revisited the place and found this to be the 

 case. I believe that if, even in 1879, I could have examined Mr. Blake's 

 specimen, I should have recognized it at once as a somewhat modified igneous 

 rock. That which I did examine (and have subsequently puzzled over many a 

 time, for I soon became less satisfied with my conclusion) was cut from a 

 junction-specimen. As in the case described above, the effects of crushing are 

 not very conspicuous ; it is somewhat banded and curiously like a rather altered 

 sediment. The micromineralogical structure is a very unusual one. Even 



