276 Report S.A.A. Advancement of Science. 



quartz, the whole of the, quartz in such a little area being in crystalline 

 continuity." This structure is very characteristic of lavas which have 

 undergone silicification, as has clearly happened in this case. 



Some specimens show in the ground mass many well-formed 

 rhombs of dolomite, which somethimes encroach on the feldspar 

 phenocrysts, but do not appear to be derived from them ; it would 

 rather seem that they have originated in the ground mass, and have 

 pushed aside the decomposing feldspar during their growth. 



Vesicles are very numerous, but are rarely more than 3 or 4 7nm. 

 in diameter. They are filled with concentric layers of quartz, 

 chlorite, calcite, and dolomite, and possibly some feldspar and 

 zeolithic minerals. 



Whatever the original character of this rock may have been, it 

 is at anv rate of volcanic origin, and in phices the structure suggests 

 that it may be something of the nature of an agglomerate, and not 

 a true lava-flow. The specimens available are hardly of sufficient 

 size to allow of the formation of a decided opinion on this point, 

 but it is at least a possibility to be considered. A careful examination 

 of the thin slices with a pocket lens yields no definite evidence. It 

 should also be borne in mind that a simulation of clastic structure is 

 often induced in viscous lavas by a kind of brecciation during flow. 

 The question of the original character and origin of these amygdaloi- 

 dal rocks must be left an open one. 



(b) The Quartz- Porphyry. 



The non vesicular type in a hand-specimen (209g, 209r, 209U, 

 211) is a rock of very crystalline appearance, and of the prevailing 

 greenish grey tint which is so common in this series. It shows 

 abundant phenocrysts of clear, glassy quartz and white, somewhat 

 opaque, feldspar. Under the microscope the porphyritic character is 

 conspicuous, and perhaps the most striking feature is the extraordinary 

 manner in which the quartz phenocrysts have been corroded by the 

 groundmass, so that they assume remarkable forms. This is a very 

 common feature of the more acid hypabyssal and volcanic rocks, but 

 is seldom so strikingly exhibited as in this case. This phenomenon 

 is apparentlv due to the varying solubility of the quartz in the magma 

 under different conditions of pressure. 



In the intratelluric stage, where the pressure is high, quartz 

 separates out in well-formed crystals, but in the effusive stage the 

 pressure is decreased, and the quartz becomes more soluble, so that 

 it is again partly dissolved by the magma. The peculiar forms are 

 due to the varying solubility of the quartz in different crystallographic 

 directions. * 



The feldspars of this rock include both orthoclase and plagioclase 

 in approximatelv equal proportions. Both occur in large, tabular 

 crystals, which have often undergone a certain amount of magmatic 

 corrosion. This is perhaps most strongly marked in the case of the 

 orthoclase. 



See Penfield, Trans. Conn. Acad., 8, 1889, p. 1 58. 



