August I, 1889] 



N A TUBE 



321 



IV. — The Period of the Younger Schists. 



Without entering into details, which are only intelligible 

 with the help of a large map and sections, and even with 

 this aid involve much disquisition of a technical kind, I 

 may briefly say that after the deposition of the limestone 

 and of the missing strata, whatever these may have been, 

 which covered them, the whole region was convulsed by 

 a series of disturbances, to which there has since been no 

 parallel within our borders. By a series of intermittent 

 movements the terrestrial crust, for thousands of feet 

 downward, over the North- West Highlands, was fissured 

 and pushed bodily westward. The various geological 

 formations of that district — Archaean, Cambrian, and 

 Silurian — were disrupted and driven over each other. 

 Thus masses of rock, not more than a few hundred feet 

 thick, were piled up so as to appear multiplied tenfold. 

 The youngest strata were doubled under the oldest, and 

 large slices of the ancient Archaean gneiss were made to 

 rest on the Silurian limestones. 



Fortunately the strongly marked characters of the 

 different members of the Silurian series, the striking 

 contrast between them and the Cambrian sandstones and 

 Archaean gneiss, and the manner in which all these 

 rocks are now laid bare on coast cliffs and rugged hill- 

 sides, have rendered possible the task of unravelling this 

 labyrinthine structure. The large maps, on the scale of 

 6 inches to a mile, on which this structure has been 

 worked out by the Geological Survey, are by far the most 

 complicated which the Survey has yet produced ; indeed, 

 I am not aware that such mapping has ever before 

 been attempted. [Some specimens of these maps were 

 exhibited.] 



On exposed rock-faces we see a thin group of strata 

 repeated again and again by small reversed faults, the 

 lower beds being made to rest on the higher till they 

 occupy a great breadth of ground, and appear of consider- 

 able thickness. Further examination will generally show 

 that they have been'all pushed westwards, and that their 

 truncated under ends rest upon a platform of undisturbed 

 rock along which they have travelled. We may further 

 observe them to be abruptly cut off at a higher level by a 

 sharp line, on which perhaps stands another series of 

 piled-up beds. This piling up and truncation of the rocks 

 is followed by a still more gigantic displacement. Lower 

 and lower portions of the geological series have been torn 

 up and thrust westward until at last the Archaean platform 

 has given 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 east-south-east. Bordering the west coast of 

 Sutherland and Ross 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. E'.ast of that strip, along 

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

 breadth, the disturbances become increasingly numerous 

 and powerful towards 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 are now flaggy schists — the 

 younger " quartzose and gneissose flagstones " of Murchi- 

 son, "the Moine schists" of the Geological Survey. 



The enormous amount of fracturing, displacing, and 

 crushing caused by these terrestrial disturbances has 

 resulted in the development 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 

 laminated slate. 



Where the feeblest amount of alteration has taken 

 place, the rock has been merely somewhat crushed, its 

 larger crystal3 or pebbles have been fractured, and the 

 separated portions have been re-cemented. 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 be- 

 come 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 effaced, 

 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 

 destruction of the various rocks, there has been a chemical 

 and mineralogical re-arrangement of their particles. Out 

 of their broken-down materials new minerals have 

 crystallized, and this process of reconstruction 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 them- 

 selves along the lines of movement in the crushed rock. 

 These lines, approximating to the surfaces of the great 

 thrust-planes, may be utterly discordant from the 

 structure-lines, such as those of foliation or bedding, in 

 the original mass. Rocks 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 distinguished 

 from Archaean schists, as to the derivation of which we can 

 only guess, and which, therefore, must in the meantime be 

 considered as original rocks. 



By the aid of the microscope, much assistance is 

 obtained in tracing out the mineral transformations which 

 have taken place in the course of this regional meta- 

 morphism. 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 1 show a selection of slides on the screen, photo- 

 graphed from thin slices of the rocks as seen under the 

 microscope. 



The importance of the discovery of this belt of extreme 

 complication in the North- West Highlands can hardly be 

 over-estimated. 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 



