Dr. C. CaUaicay— Reply to General McMahon. 



321 



with crushed granite. The chlorite and iron-oxide are seen ^ to 

 be creeping in between the fragments of granite, and forming 

 a matrix to them. Another slide, taken from within an 

 inch, shows the same thing; but the signs of shearing, as dis- 

 tinguished from mere crushing, are evident. Many other slides, 

 selected first at intervals of an inch or two, and then at greater 

 distances, exhibit more and more clearly the laminated structure of 

 the ordinary gneiss, as the shearing becomes more even and uniform. 

 Thus we see produced a gneiss in which lenticles of quartz-felspar 

 granite are interlaminated with chlorite and iron-oxide. Here and 

 there patches of black mica appear in the chlorite, especially round 

 particles of the opaque iron-oxide. In the same locality, at a little 

 distance, the chlorite is largely replaced by the mica. The evidence 

 for the origin of this mica appears to me irresistible. It must have 

 come from either the infiltrated products, or from the granite, or in 

 part from both. But it could not have come from the granite, for that 

 contains a mere trace of iron-oxide and magnesia. It must,^ there- 

 fore, have originated from the chlorite and iron-oxide, with, no 

 doubt, the assistance of potash from the partially decomposed granite. 

 But I will give my critic another case. I will put it in a semi- 

 diagrammatic form, merely remarking that it represents what occurs 

 in countless sections in the Malvern Hills. The distance from 

 A to C is usually a few feet or yards. 



The gradation between A and C is clearly seen both in the field 

 and in microscopic slides, of which some hundreds bearing upon 

 this point have been examined. It is apparent in the felspars, 

 whose changes can be followed step by step. It is equally clear in 

 the basic minerals, the hornblende decomposing into chlorite, iron- 

 oxide, and sometimes epidote, and the chlorite becoming banded 

 with biotite, and, within the complex, being largely replaced by it. 

 It appears to me inevitable that either the biotite has been produced 

 out of the hornblende through the intermediate form of chlorite, or 

 the hornblende has been produced out of the biotite through the 

 same intermediary. But if the latter alternative be accepted, how is 

 it explained that an unsheared diorite is produced out of a sheared 

 micaceous rock ? 



General McMahon will perhaps suggest that there must be a 

 break somewhere ; that the biotite is decomposed to chlorite, and 

 the hornblende is an independent mineral. This view is negatived 

 by the fact that the hornblende and the chlorite pass into each other. 



be that both the hornblende 



A more reasonable suggestion would 



DECADE IV. — VOL. I. — NO. VII. 



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