1889.] on the Highlands of Scotland arid the West of Ireland. 539 



way, and masses of it, many hundreds of feet in thickness and many 

 miles in length, have been driven over the younger formations. The 

 horizontal distance to which this removal has reached can sometimes 

 be shown to have amounted to at least ten miles ; perhaps it may have 

 been sometimes even greater. 



In studying this complicated system of dislocations we soon meet 

 with evidence that the movements were not all effected at one time, 

 but that on the contrary they took place at intervals, the earlier 

 being disrupted by the later. The lines of maximum thrust override 

 those of lesser size, and the most easterly of these lines passes suc- 

 cessively across all the others till it rests directly on unmoved rocks. 

 The period of terrestrial disturbance was probably a prolonged one, 

 and this inference is strengthened by other evidence to be afterwards 

 adduced. 



The direction of movement has been on the whole from the 

 E.S.E. Bordering the west coast of Sutherland and Eoss there is a 

 strip of ground about 10 or 15 miles broad and some 90 miles 

 long, in which the rocks have not been displaced. East of that strip, 

 along a belt of dislocation varying up to five or six miles in breadth, 

 the disturbances become increasingly numerous and powerful to- 

 wards the interior, until at last a gigantic thrust-plane is encountered, 

 above and beyond which the rocks have been so crushed and altered, 

 that it is for the most part no longer possible to tell what their 

 original character has been. They a^e now flaggy schists — the 

 younger " quartzose and gneissose flagstones " of Murchison, " the 

 Moine schists " of the Geological Survey. 



The enormous amount of fracturing, displacing and crushing 

 caused by these terrestrial disturbances has resulted in the develop- 

 ment of regional metamorphism on an extensive scale. Every stage 

 can be traced from a sandstone or conglomerate into a perfect schist, 

 and from the most typical coarse Archaean gneiss into a fine lami- 

 nated slate. 



Where the feeblest amount of alteration has taken place, the rock 

 has been merely somewhat crushed, its larger crystals or pebbles 

 have been fractured, and the separated portions have been recemented. 

 A further stage is shown where the fine material of the rock has 

 been more comminuted and has been drawn out round the flattened 

 and elongated crystals or pebbles. The latter give way in proportion 

 to their power of resistance. The felspars and hornblendes are first 

 left as "eyes" and then crushed down till they disappear in the 

 general matrix. The harder quartz-pebbles of the conglomerates 

 have resisted longer; but they too, in the planes of great movement, 

 are found to be pulled out to twice or four times their length, or to 

 be flattened out into mere thin plates like pennies. One of the most 

 singular proofs of this internal movement of the component particles 

 of even so obdurate a rock as quartzite is shown by the deformation 

 of the worm-tubes. As these tubes come within the influence of the 

 movement their vertical position changes into an inclined one, and they 

 Vol. XII. (No. 83.) 2 o 



