574 Correspondence — Major- General McMahon. 



must be left for further work. On one, however, I feel quite clear, 

 namely, that the relations of the rocks have been completely mis- 

 understood by Mr. Somervail. 



(4). "On the Nature and Origin of the Banded Structure in the 

 Schists and other Eocks of the Lizard District." l As this subject 

 will form an important part of our paper, I content myself with 

 observing that I can find no ground for Mr. Somervail's hypothesis 

 of segregation, as he applies it. I question both the accuracy of his 

 statements and the validity of his inductions. Doubtless before 

 writing upon these difficult subjects, Mr. Somervail has trained 

 himself by careful study both of rock-structures under the micro- 

 scope, and of rock-relations in less complicated districts of other 

 regions: but if so, I am utterly at a loss to understand his principles 

 of interpretation and his methods of reasoning. 



T. G. Bonney. 



BANDED ROCKS OF THE LIZARD. 



Sir, — Mr. Somervail in his paper on the Lizard rocks, published 

 in your last issue, advances the theory of segregation to explain all 

 the phenomena displayed by the eruptive rocks of that interesting 

 localit}', but he does not favour us with any evidence in support of 

 his theory, and he omits to explain facts that seem incompatible with 

 it. That such rocks as peridotite, gabbro, diorite, basaltic, and 

 felspathic traps, and granite — rocks of well-defined species differing 

 from each other in mineralogical contents, structure, and chemical 

 composition (points that imply genetic differences) — should be formed 

 on the spot by segregation from a "common magma," is sufficiently 

 startling to the petrologist; but when we find, as competent ob- 

 servers bave found, that these rocks cut each other in well-marked 

 dykes following each other in a regular sequence, and that each of 

 the principal intruders carries along with it sharp fragments of the 

 rocks through which it has intruded, the trypothesis involves the 

 rejection of every canon of interpretation hitherto relied on by field 

 geologists. 



When one sees diverse igneous rocks cutting across each other in a 

 way that implies differences in their order of eruption ; and when one 

 finds the lines of demarcation between these successive eruptions so 

 sharp that even thin slices examined under the microscope show a 

 sudden transition from a rock of one chemical and mineralogical 

 composition to another of different chemical and mineralogical com- 

 position, it seems as unreasonable to a petrologist to attribute the 

 formation of these definite and distinct species to segregation in situ 

 as it would be to attribute the jaw-bone and teeth of a well-known 

 quadruped, found in a bed of marl, to the fortuitous segregation of 

 the carbonate of lime. 



The above-mentioned rocks not only cut each other with a definite 

 sequence, but they preserve their individual characteristics, whether 



1 Geol. Mag. 1890, Dec. III. Vol. VII. p. 515. 



