538 Mr. Archibald Geikie [June 7, 



geological positiou of which there is any certainty. At present we 

 know absolutely nothing of other sedimentary strata which followed 

 that limestone. That such strata continued to be deposited is certain, 

 for the changes which the quartzites and limestones have undergone 

 could not have taken place save under the pressure of a thick mass of 

 overlying material. But this superincumbent mass has been entirely 

 obliterated in the extraordinary series of terrestrial movements which 

 I have now to describe. 



IV. The Pekiod 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 i^ushed bodily westward. The various 

 geological formations of that district — Archaean, Cambrian, and 

 Silurian — were disrui^ted and driven over each other. Thus masses 

 of rock, not more than a few hundred feet thick, were jiilcd 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 Archasan 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 laby- 

 rinthine 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 exjjosed 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 considerable thickness. Further examination will generally 

 show that they have been all pushed westwards, and that their trun- 

 cated under ends rest upon a platform of undisturbed ruck 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 ujd and trunca- 

 tion of the rocks is followed by a still more gigantic disjDlacement. 

 Lower and lower portions of the geological series have been torn 

 u]) and thrust westward until at last the ArchsGan platform has given 



