BY THE REV. W. B. CLARKE, M.A., F.G.S., &c. 269 



Daubree calls imbibition, i.e., when a rock is in a condition to be 

 compared with a sponge, so that mineralised salts or fluids can 

 permeate it. Other changes take place by the more direct 

 agency of chemical, galvanic, electric, or other mysterious forces, 

 among which heat plays an important part, but not to the extent 

 which is generally imagined. Pressure also acts powerfully 

 and entirely alters the condition of rocks. It is not surprising 

 that hasty observations should have led some geologists to 

 attribute effects to heat, which we may see, perhaps, by and 

 by, it is hardly capable of effecting alone. Thus trappean 

 rocks have oftentimes been charged with causing great eleva- 

 tions and disruptions when they have been incapable of pro- 

 ducing any such effects, so far as inquiry shows them to have 

 merely flowed through fissures caused by other agency, and to 

 have produced effects somewhat different from those attributed 

 to them. 



When two masses of different kinds of rock, or of different 

 composition, structure, origin and age, are in contact, there is 

 often noticed a mutual change near the planes of contact ; this 

 transmutation Delesse calls normal metamorphism. When these 

 changes are evidently traceable to adventitious causes, such as 

 irruptions of heated matter, the transmutations are denominated 

 special or abnormal. In one or other of these ways large masses 

 of strata are changed; and although the ancient slates and 

 associated rocks known as metamorphic, betray a transmutation 

 on the largest scale, the phenomena of such change are widely 

 apparent in the Tertiary, Cretaceous, and Jurassic formations of 

 the Pyrenees, the Alps, the Carpathians, Turkey, the Caucasus, 

 Armenia, and Himalaya ; and, what is remarkable, in the 

 greater mountain chains the transmutations are more striking 

 than in the smaller ranges of the same age, as is evidenced in the 

 Hartz, Wurtemburg, the Jura, Aragon, New Castille, and in 

 other mountains where the natural features are on a smaller 

 scale. D'Archiac shrewdly points out that these variations are 

 not necessarily due to the presence of igneous rocks, because 

 transmutation has gone on in the greater chains oftentimes far 

 from such igneous matter. 



It appears, nevertheless, certain that where transmutation 



