Dr. C. Callaway — Diorite and Acidic Gneiss. 221 



gradation between two kinds of rock proves notliing* as to tlie 

 genetic connection between them. In an igneous magma, for 

 example, one part of a mass may be acidic and the other basic, 

 and they may grade imperceptibly into each other. 



What I mean will be best elucidated by an illustration : A piece 

 of beef is roasting before the fire. The central part of the mass is 

 raw and the outer layers are more or less cooked. By cutting, into 

 the meat, we see the gradation between the raw and the cooked 

 parts. We also see the cause of the change produced. The proof 

 of the roasting of the meat is therefore complete. 



This illustration represents the kind of evidence for tbe production 

 of acidic gneiss from diorite offered us by the Malvern rocks, and 

 it hardly exaggerates its clearness. We can observe in the rocks 

 the crushing up of the constituents increasing in intensity, and 

 parallel with this change is a mineral transformation. Or the 

 change in the minerals gradually comes in as we approach a plexus 

 of granite-veins. Or both of these causes of alteration may be 

 combined. The evidence of increasing heat as we penetrate a 

 shear-zone is seen in the recementation of minute fragments and 

 shear-lenticles, or in the progressive clearness of the minerals, or 

 in both characters united. Further proof of secondary action is 

 furnished by the relations of the minerals in the completed gneiss, 

 in which we sometimes observe that mica, felspar, and quartz are 

 all moulded upon calcite, which in these rocks is unquestionably 

 a secondary product. 



The other objections which have been urged against my conclusions 

 on the origin of the acidic gneiss are chiefly of a chemical nature. 

 The elimination of magnesia and alumina, especially the latter, and 

 the production of biotite out of chlorite, iron-oxide, and potash, are 

 apparently regarded as so improbable that no amount of evidence 

 furnished by field-sections or microscopic slides can establish their 

 reality. This position I contest. I respectfully urge that the 

 chemical theories must give way to the geological facts. If a rock 

 containing 18 per cent, of alumina and 7 per cent, of magnesia 

 has been converted into one in which the percentages are respec- 

 tively 9 and 0-5, then alumina and magnesia Jiave been eliminated. 

 The analyses of Mr. Player prove that the diorite and the acidic 

 gneiss respectively contain the proportions here stated ; and, since 

 the one has been changed into the other, we are driven to conclude 

 that the elimination has taken place. 



Magnesia and alumina, especially the latter, are no doubt very 

 stubborn things to get rid of; but we have no right to assume that 

 they cannot be eliminated under any circumstances. Laboratory 

 experiments are of very little use to us here. We are quite 

 incapable of determining beforehand what may or may not take 

 place in a granite-diorite shear-zone, where the rock is crushed 



1 Since the above was written, the "Annals of British Geology" for 1893 has 

 appeared, in which the Eev. J. F. Blake (p. xxi) falls into the astonishing error of 

 attributing to me the precisely opposite opinion. It is not surprising that new 

 views should make slow progress when they are thus misunderstood and mis- 

 represented. 



