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



Although I provisionally cancelled one of these thi-ee groups, the 

 " talco-micaceous," my doing so, as I then stated, would not 

 materially affect the reasoning in that paper, neither will it do so 

 in the present, even if I should be wrong on this point. 



The gist of some of my remarks was also meant as introductory 

 to an attempt in the present paper to show that the origin of the 

 banded structure pervading certain of these Lizard rocks might be 

 explained on the ground of segregation, before or during the cooling 

 of the common magma out of which they seemingly had been formed. 

 The fact alone of this magma giving rise to such a diversity of rocks 

 which, although in certain parts of the field isolated from each 

 other, are yet in other parts not only associated and blended together 

 in a somewhat massive way, but in other districts are mingled with 

 each other on a much smaller scale, and with a greater degree of 

 symmetry, so as to pass by easy transition into what is properly 

 termed the banded structure. These facts alone of themselves seem 

 to supply an intelligible basis, or starting-point, for an explanation of 

 this phenomenon — of course confining the term "banded " to those 

 more strictly symmetrical portions of this structure now to be 

 described. 



I. Nature of the Banded Structure. 



Although this paper is principally confined to the banded struc- 

 ture in the rocks included in the triple or double group of schists 

 and rocks, it may yet embrace all the other rocks of the area, as 

 all of these are more or less banded in one form or another, and 

 the same explanation seems to me to serve for the whole, in the 

 wide and proper sense of the term. 



In the double group of schists (and rocks), viz. the "granulitic" and 

 " hornblendic " (between which I have divided the talco-micaceous) 

 are rocks with a wide variation in their banded structures, especially 

 in the granulitic group. In this group it assumes many different 

 forms, ranging from well-defined banding of a more or less regular 

 and persistent horizontal type, to highly irregular bands thinning and 

 swelling out again. At other times the bands are lenticular and run 

 in an oblique direction, and even the materials which form the banding 

 in the base of the rock are sometimes found in isolated patches 

 without any visible connection between them. General McMahon 

 has given a veiy succinct description of the banding in this group 

 which is as follows : " The rocks of this group are usually spoken 

 of as a banded group, and appearances often support the idea that 

 the quasi-banding is a regular banding parallel to the bedding. 

 When the eye of the observer becomes accustomed to these rocks, 

 however, the banding is seen to be extremely irregular in character. 

 Not only do some of the bands dwindle into strings and die out in 

 the direction of their length, but they inosculate, bifurcate, and 

 entangle themselves into a complicated network of meshes, and 

 they swell in places into broad lacunas, which, in their turn, throw 

 off fine veins in all directions. Even where these bands are apparently 

 most regular, they will often be found, when followed up, to behave 

 like injections." 



