510 A. Somervail — Banded Rocks of the Lizard District. 



and another of granulitic materials alternating with each other, often 

 with much uniformity, in their respective thicknesses, and this too 

 continued over considerable areas. 



2. This theory does not explain the frequent transition of these 

 bands into each other, which, although comparatively distinct and 

 well-defined from each other in some areas, and portions of the same 

 mass of rock, are nevertheless, in others, marked by a passage of 

 their mineral constituents. 



3. The theory does not explain the banding — most frequently 

 parallel — in those masses of nearly homogeneous granulitic rocks, 

 where the banding is entirely made up of the slightly varying quartzo- 

 felspathic mineral constituents which form them, and not of the 

 widely contrasted bands of dioritic and granulitic minerals ; these 

 rocks not coming under what is designated as an igneous complex. 



Injection. — This theory has been advanced by General McMahon 

 more particularly to account for the banding in the " granulitic " 

 group, a group, as already stated, considered by him to be made up 

 of volcanic ashes and lavas, into which have been intruded diorites 

 and subsequently other porphyritic diorites, and by granites still later, 

 each in its turn injecting the other. It is, however, to the last of 

 these, the injection of the gi*anite, that, according to General McMahon, 

 the banding is due, or mainly due, at least it is made the prominent 

 factor in his explanation. He remarks that, " the process of injection 

 was doubtless aided by the partial plasticity of the dioritic rocks — a 

 plasticity induced probably by the neighbourhood of igneous masses, 

 for it seems to have been local in its development. It would also 

 have been aided in the case of consolidated ash-beds — especially in 

 submarine ash-beds — by the planes of sedimentation, and in the case 

 of diorite intruded as sheets by the foliation developed in a direction 

 parallel to the bedding. The existence of such planes of weakness 

 would explain why the injected granite displayed a tendency to 

 follow lines parallel to the bedding, and in doing so produced the 

 superficial appearance of banding." 



This explanation of Genei'al McMahon's certainly much better 

 meets the difficulty or objection just urged against Mi*. Teall's 

 views, which afford no explanation of the symmetry of the banding, 

 to which attention was drawn ; indeed, according to the theory of 

 the latter (if I understand it aright), it would be entirely destructive 

 of this parallel or symmetrical structure, and not the cause of it. 



As a whole, however, the " injection " theory seems to me to 

 fail even in the one direction in which it has been applied, while 

 it offers no adequate explanation of the banded structure in a 

 general sense. 



To whatever cause, then, the banded structure may ultimately be 

 assigned, I think it sufficiently clear that all of these three modes 

 of explanation completely fail when applied to these Lizard rocks. 

 First, the sedimentary, because it is at once negatived by the igneous 

 origin of the rocks in question, no tuffs as yet having been recog- 

 nized. Second, the deformation theory, because it never could 

 produce the linear arrangement or parallel structure as we find it 



