ANIiTIYEKSART ADDRESS OF THE PRESIDENT, lOI 



specimen represents the condition of the Hebridean in the fold- 

 fault at Loch Emboli. 



There may be, of course, various intrusive igneous rocks in this 

 " newer series " of Loch lEaree which have been crushed up like the 

 gneisses and schists ; but I think that the latter certainly pre- 

 dominate. It is also possible that portions of Paloeozoic rocks may be 

 faulted or folded in ; but I feel csrtain that of them there is nowhere 

 a great quantity, and that I saw none. Moreover I fully believe 

 that, except very locally, there would be no real difficulty in 

 distinguishing between the crushed gneisses and schists and the 

 compressed Torridon Sandstone, quartzites, or other Palaeozoic 

 sedimentary rocks, provided only they be studied both in the field 

 and with the microscope, and that the observer has duly qualified 

 himself for the task by previous education. 



Among the effects of these great pressures will undoubtedly be an 

 elevation of the temperature of the rock, and this will facilitate 

 mineral changes. But I think that this factor in metamorphism has 

 been overestimated by some authors, and that it is of secondary rather 

 than of primary value. Pressure alone, as shown by Dr. Sorby and 

 others, is a most important agent in promoting the solution of 

 minerals ; and, when this ceases to act, deposition and crystallization 

 may take place. Strain or pressure may be relieved, in the course 

 of ages, by fracture and movement, by the settlement of the mass, 

 and other causes. The excavation of valleys and denudation 

 generally must produce important effects in this respect. 



Co ntact-Metamorphism . 



Rocks affected by contact-metamorphism (by which term we 

 mean the alteration produced upon sedimentary rocks by the intru- 

 sion into them of igneous masses) while differing materially from 

 those commonly called metamorphic — so much so, that the separation 

 under the microscope of the one from the other is generally ex- 

 tremely easy — nevertheless present some suggestive resemblances. 

 If we leave out of consideration the less extreme cases of contact- 

 metamorphism, such as we find in the vicinity of most dykes and 

 small intrusive masses, or where proximity to the surface of 

 the earth has permitted a more rapid escape of heat, such, for in- 

 stance, as abound in the Carboniferous series of Fifeshire ; if we 

 also pass by, as of comparatively simple character, the change of 

 sandstone into quartzite, and the conversion of ordinary limestone 

 into crystalline marble, and confine our attention to the argilla- 

 ceous rocks, as of more complicated chemical composition, and to 

 the cases where they are in proximity to large deep-seated intrusive 

 masses, we find changes analogous to those which have been pro- 

 duced in the true schists, though, as I have said, presenting marked 

 differences in structure. This analogy is not surprising ; for pro- 

 bably the ageuts of change have been identical, viz. heat, pressure, 

 water, working conjointly and for a considerable time. 



I may illustrate the nature of the changes thus produced, by a 

 brief sketch of the alteration in the Skiddaw slate a.s it approaches 



VOL. XLII. i 



