132 



REV. E. HILL AND PROF. T. G. BONNET ON THE 



Fig. 4. — Relations of gneis- 

 soid, Sfc. rocks a little 

 above the sJiore in Port a 

 la Jument. (iV. of the 

 path.) 



feet, interbedded with the compact granitoid component. In places, 

 however, a fairly coarse granitoid rock appears to break in among 

 the others. Beneath the whole is a low terrace of the basement- 

 gneiss. The annexed diagrams will serve to show the association in 

 one of the more uniform parts (fig. 4) and the mode in which 



gneissoid or granitoid rocks enfold 

 the lumps of hornblende in one of 

 the more irregular parts (fig. 5). 



(c) Conclusions. — From these ob- 

 servations, and others which it would 

 be tedious to describe in detail, we 

 conclude that a basic rock, now 

 represented by the hornblendic rock, 

 is the oldest. This was shattered 

 by the intrusion of a rather acid rock, 

 represented by the veins of felspathic 

 aplite. By this it was sometimes 

 softened and drawn out, and portions 

 of it (probably small in size) occa- 

 sionally were actually melted down. 

 In this case the constituents derived 

 from the hornblende, when the mass 

 consolidated, combined with some of 

 those in the material of the aplite, 

 and thus much of the biotite was 

 produced.^ 



The whole mass probably continued 

 to move, slightly and slowly, during 

 crystallization, and to this both the 

 macroscopic banding of the gneiss 

 and its peculiar microscopic structure 

 are due.^ In short, we find here 

 (with certain obvious but theoreti- 

 cally unimportant differences) another 

 instance of the process which we are 

 now satisfied must have occurred in 

 more than one banded rock at the 

 Lizard. 



But how is this gneissic series 

 (which seems to be not easily sepa- 

 rable from the ordinary hornblende- 



1. Basemen t-giieiss. 



2. Granitoid rock, parted from 



(1) by a crack or a horn- 

 blendic (?) seam. About 8 

 inches. 



3. Banded biotite-gneiss, rather 



quartzose, about 10 or 12 

 inches. 



4. Granitoid rock, about 15inches. 



5. Banded biotite (or hornblende) 



gneiss, with occasional len- 

 ticular masses of hornblende- 

 rock and bands of greyish 

 red gneissoid or granitoid 

 rock. It forms one mass, 

 extending upwards for some 

 yards. 



schist and gneiss) related to the 

 basement-gneiss? Is the last only the main mass of the magma, 



^ K. A. Lossen (Congres Internat. Geol., Comptes Rendus de la 4™^ Session 

 (1888), p. 184) speaks of alteration-products rich in biotite occurring in diabases 

 (at the expense of pyroxenes) when diabase has been broken into by granite or 

 gabbro. Also C. Callaway (Quart. Journ. Geol. See. vol. xlv. (1889) p. 480) 

 asserts the formation of biotite from hornblende through chlorite, as the result 

 of shearing in a hornblendic granite ; but obviously these cases are different from 

 the mode of production which we describe. 



2 Namely, not only the arrangement but also the forms of the grains of quartz 



