480 PROF. T. G. BONNEY AND MAJOR-GEN. C. A. M*^MAHON 



of the above explanations to them. The ordinary cases, where 

 the rock is foliated rather than banded, and the slabby bedding, 

 which is commonly so marked a feature as to be the tirst thing 

 that attracts the eye as the mass is approached, might be accounted 

 for, like the structures in the " newer gneiss " series of Glen Logan 

 and its vicinity, by the effects of shearing movements during a long 

 continued process of thrust-faulting ; but, in applying this hypo- 

 thesis to some of the structures which are more especially 

 suggestive of stratification, we were always encountered by diffi- 

 culties which we failed to overcome. Again and again the gliding- 

 planes, which we had devised in order to explain the oblique 

 disposition of the apparent stratulm in the rock, were interrupted 

 by some unbroken band which either forbade the idea of any dis- 

 placement, or demanded for its manufacture a contradictory set of 

 movements. In these cases the " fluxion hypothesis " also landed 

 us in similar difficulties. Thus, although our reasons cannot be 

 fully appreciated by those who have not followed our steps, we are 

 at present unable to suggest any form of mechanical disturbance as 

 a complete explanation of the more banded members of the Horn- 

 blendic Group, and think that for these the stratification of an ash 

 (perhaps by the intervention of water *) is the better " working 

 hypothesis." 



Subsequently, of course, there must have been almost complete, 

 if not quite complete, rearrangement of the constituents. The ash 

 originally must have consisted of more or less fragmeutal felspar, 

 augite, iron oxide, and possibly olivine, with bits of more or less 

 scoriaceous tachylyte or magma-basalt. Of the latter, at any rate, 

 every trace has disappeared, the constituents have separated as from a 

 molten mass, and the whole is a crystalline mixture of felspar, horn- 

 blende, &c.t In this hypothesis there are also difficulties, so that, 

 until further evidence be discovered, it must be regarded as only 

 tentative, for we now feel con\dnced that some members of the 

 group were originally dolerites, and some structures are due to 

 fluxion. Moreover, it must not be forgotten that the basic member 

 of the overh'ing Granulitic Group often differs little from the more 

 hornblendic part of the present one. The former appears to have 

 been raised to a high temperature after it had at least begun to 

 consolidate. It is then probable that the underlying rock was not 

 less affected, and important changes may thus have been brought 

 about. 



(3) The Micaceous Group. 



This — the talco-micaceous group of De la Beche — was retained 

 by Prof. Bonney because of the presence of a mica-schist, and some 

 other non-hornblendic rocks, among the green schists. The last 



* Because by the action of currents the materials would be to some extent 

 separated in accorrlance with their specific gravity, and their deposition at any 

 spot would be varied by the constantly changfng velocity of flow. 



t See, for a suggestion of the process, M'^Mahon, Quart. Journ. Geol. Soc. 

 vol. xlv. (1889) pp. 525-531. 



