540 ^^' Archibald Geikie [June 7, 



becoroe gradually flatter and more drawn out till at last, before they 

 cease to be traceable, they appear as mere long ribbons on the surface 

 of the rock, which then becomes a quartz -schist. Along the planes 

 of intense crushing the original structure of a rock is entirely eifaced, 

 its crystals or grains are ground into fragments, and it acquires a 

 streaked laminated structure like a shale or slate. 



But for the most part, concomitant with the mechanical destruc- 

 tion of the various rocks, there has been a chemical and mineralogical 

 re-arranf»ement of their particles. Out of their broken-down 

 materials new minerals have crystallised, and this process of recon- 

 struction has, in the most thoroughly altered masses, proceeded so far 

 that the whole new structure is now crystalline. In this manner, 

 mica, quartz, felspar, hornblende, and other minerals, have been 

 developed, and have arranged themselves along the lines of movement 

 in the crushed rock. These lines, approximating to the surfaces of 

 the threat tlirust-iilanes, may be utterly discordant from the structure- 

 lines, such as those of foliation or bedding, in the original mass. 

 Eocks of this character are true schists, and I know of no internal or ■ 

 external signs by which, apart from field-evidence, they are to be 

 distinf^uished from Archaean schists as to the derivation of which we 

 can only guess, and which, therefore, must in the meantime be con- 

 sidered as original rocks. 



By the aid of the microscope, much assistance is obtained in 

 tracin^y out the mineral transformations which have taken place in 

 the course of this regional metamorphism. To show the larger features 

 of the change, so far as they can be judged of in hand-specimens, I 

 exhibit on the table a series of pieces of the crushed gneiss, quartzite, 

 and conglomerate ; and to illustrate the internal changes I show a 

 selection of slides on the screen, photographed from thin slices of 

 the rocks as seen under the microscope. 



The importance of the discovery of this belt of extreme compli- 

 cation in the north-west Highlands can hardly be overestimated. It 

 gives us the key to the geological structure, not only of the Highlands, 

 but of all the areas of younger crystalline schists in our own area, 

 and will doubtless be found to explain much in the geological struc- 

 ture of Scandinavia. The lines of maximum thrust-planes can be 

 followed for 100 miles, from the north of Sutherland into Skye; but 

 this is only a small part of their extent. They can be picked up 

 again in the west of Mayo and Donegal, a total distance of some 

 400 miles. That similar lines of movement have aifected Scandi- 

 navia and produced the distinctive strike of the rocks there can 

 hardly be doubted, so that the total length of disturbed country in 

 north-western Europe probably exceeds 1600 miles, trending in a 

 general north-north-east direction. 



How far the influence of the great terrestrial movements extended 

 eastwards from what now appears as the belt of maximum dis- 

 turbance, and what effect it had upon the configuration of the surface, 

 are questions to which as yet no satisfactory answer can be given. 



