Felslte Dyke, Mont Park. 6P 



distinct platy layers, disposed parallel to the wall of the fissure- 

 Its eastern face is, as already noticed, polished, and faintly striated.. 

 The dynamical movement to which the rock luis been subjected 

 probably accounts for the laminated appearance. 



This rock in thin sections (Plate VI., figs. 1 and 2) under a 

 moderately high power, is seen to consist of fine felspathic material 

 closely felted, with larger fragments of altered felspars and 

 extremely angular fragments of quartz. These larger fragments; 

 are arranged rudely parallel to the walls of the fissure. The very 

 fine felsitic constituents, by being impregnated with limonite, are 

 not conspicuously anisotropic under crossed nicols. but the edge of 

 this thin band of rock has lost its iron, being very clear in excessively 

 til ill .sections, ami giving striking polarisation effects. A still 

 hiirher magnification I'eveals the presence of minute fi-agments of" 

 pale green augite, occasional red-brown rutiles, and numerous: 

 rounded crystals of zircon. Throughout the rock there are ragged 

 fragmentary folia of biotite, whilst disseminated here and there- 

 may Ije seen limonitic granules, probably decomposition products, 

 restdting from the alteration of 1)iotite. and other ferro-magnesiant 

 minerals. 



The rock may be described as a fragmental felsitic rock resem- 

 bling an ash, but its origin is a difficult problem. There are three- 

 posible solutions :--(!) A thin intrusion of a felsitic character much 

 later than the main dyke itself; (2) a true acid volcanic ash; and 

 (3) a fragmental rock due to the grinding of the walls of the fault 

 fissure, the constituents of the rock consequently being derived 

 largely from igneous rocks. 



Concerning (1), it is difficult to understand the completely 

 crushed quartz, and the entirely fragmental character of the matrix, 

 for so much dynamical stress would surely result in chemical changes 

 within the rock itself. With regard to (2). the microscope rather 

 favours this suggestion, but it is difficult to account for such a 

 thin band of volcanic ash in such a position, considering that it 

 is clearly not interbedded with the Silurian sediments of the area, 

 and that it is extremely unlikely that a narrow fissure would remain 

 open, and become filled with pyroclastic material to such a depth 

 as this fissure must have po.ssessed, in view of the very considerable- 

 denudation to which the rocks must have since been subjected. The 

 most favourable solution appears to be (3). but whilst we incline 

 towards this idea, Ave think the origin of the rock must be left an 

 open question for the present. 



We are indebted to Mr. D. J. Mahony. M.Sc, for assistance in^ 

 elucidating the petrology of the rocks examined. 



