A. Somervail — Banded Rocks of the Lizard District. 513 



is quite apparent. The parallel or banded structure, a priori, would 

 be as likely a one as any. 



Dr. Sterry Hunt, in contributions to the American Journal of 

 Science for March and July, 1864, gives several instances of banding 

 in rocks which, according to his own descriptions, seem to me to be 

 due to segregation. Describing a granitoid dolerite at Mount Royal, 

 he says it consists of "Mixtures of augife with felspar in parts of 

 which the felspar predominates, giving rise to a light greyish rock. 

 Portions of this are sometimes found limited on either side by hands 

 of nearly pure black pyroxenite, giving at first sight an aspect of 

 stratification. The bands of these two varieties are found curiously 

 contorted and interrupted, and seem to have resulted from move- 

 ments in a heterogeneous pasty mass, which have effected a partial 

 blending of an augitic magma with another more felspathio in its 

 nature." In another communication by the same author to the 

 Boston Society of Natural History, January 7th, 1874, he describes 

 an eruptive diorite from Lambertville, New Jersey, " which is con- 

 spicuously marked by light and dark bands due to the alternate 

 predominance of one or the other of the constituent minerals." The 

 same author, again, in his "Chemical and Geological Essays" 

 (Granites and Granitic Vein-stones), gives instances of a banded 

 structure in granites, and granitic dykes. All of which instances 

 can only be explained I think by segregation. 



While regarding what I would term the normal or true banding 

 as of segregation origin, I do not wish to overlook what I myself 

 believe to be a fact, viz. that certain types of the banding in some 

 of these Lizard rocks are due to subsequent causes, the product 

 of severe mechanical pressure or crushing. Such banding, or 

 perhaps more properly speaking, foliated structure, as occurs 

 among some of the hornblende schists, and also in certain portions 

 of the gabbro, I think may be due to this cause ; but, after all, this 

 is still the product of segregation, secondary segregation we may 

 call it, produced by the heat being sufficiently great to admit of 

 partial fusion, and of certain of the minerals in a greater or lesser 

 degree to re-arrange themselves. Under these circumstances 

 secondary segregation would seem to me to give rise much more 

 readily to certain forms of lenticular foliated structure, rather than 

 to the parallel banded structure proper, which I think is due to 

 original, or primary segregation, while the rock was in a complete 

 state of fusion. 



There are other forms of what might be spoken of as banding, 

 which, however, are only pseudo-bands which seem merely due to 

 the development of secondary or alteration products between the 

 planes of cleavage, such as are frequently met with in the horn- 

 blende schists. These and certain other similar types more properly 

 fall under the term cleavage foliation, rather than under that of 

 banding-. 



DECADE III. VOL. VII. NO. XI. 33 



