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



product of one cooling mass arranging and separating itself into 

 distinct layers as before described. 



An excellent instance of what I regard as segregation bands 

 in the hornblende group may be studied in a large boulder resting 

 in the small cove immediately to the west of Lower Balk Serpen- 

 tine Quarry. This mass presents a parallel banded structure made 

 up as follows. There are broad and narrow bands of nearly pure 

 pale-coloured felspar alternating with thinner bands of nearly pure 

 dark hornblende more crystalline than the base of the rock. What 

 looks at a little distance like other broad bands of the pale felspar 

 are found on closer inspection to be made up of very thin deposits 

 of that mineral set closely together, some of which are not persistent 

 for a small space, but are again continued without any interruption 

 in their parallel course. At intervals along with these are bands of 

 varying width of the ordinary hornblende, some of which are highly 

 porphyritic, the long axis of the felspar crystals being parallel to 

 each other, and in the direction of the bands themselves, as if that 

 mineral in places was not sufficiently abundant to be drawn out into 

 the form of continuous bands. 



All these varieties of banding in this one mass of the hornblende 

 series was most convincing to my mind that the felspar had arranged 

 itself, as also the more crystalline hornblende, by segregation during 

 cooling. 



Some of the difficulties in the way of an explanation of the banded 

 structure, pointed out by General McMahon, such as where in some 

 instances the bands " inosculate, bifurcate and entangle themselves 

 into a complicated network of meshes," and in other cases given by 

 myself where filaments or thread-like processes, and broader strings, 

 cross at various angles from one band to another, are all readily 

 enough explained on the grounds of segregation while the mass of 

 the rock was in a molten or plastic state, admitting of the various 

 movements of its several constituents. 



The banded structures in the gabbro and serpentine are equally 

 well accounted for by segregation, while on the ground of deforma- 

 tion it is barely possible, more especially in the latter rock. 



IV. Concluding Eemarks. 



Although we are quite unaided, as far as I am aware, by the 

 results of any experiments such as might be made by melting the 

 basic and acid rocks together, as, for example, diorite and granite, 

 and allowing the molten magma slowly to cool again, in order to 

 ascertain what arrangement its several constituents might take, 

 effected no doubt by their different cooling points, chemical affini- 

 ties, and specific gravities, still, we can infer from reflection, and 

 from certain rocks presented to our observation in the field, that 

 under certain conditions, such as unequal and different rates of 

 cooling of the various mineral constituents, abnormal forms of 

 crystallization would arise. That these and other conditions would 

 produce various structures such as the separation by segregation of 

 the various minerals in masses of more or less definite arrangement 



