C— GEOLOGY. 69 



him how he had been forced to recognize a zone, situated on the mountain 

 front, where older rocks are habitually overthrust upon younger. His 

 knowledge of the country was so thorough that he did not merely indicate 

 the position of the postulated thrust near Quebec, but laid down its 

 course all along its Canadian outcrop from Lake Champlain to the 

 extremity of Gaspe. On this account the Champlain-St. Lawrence thrust- 

 zone is often spoken of as the Logan Line. 



Logan was, of course, only applying a familiar principle ; for, in the 

 States, thrusts had been described by the brothers Rogers as early as 1842, 

 and, in the Alps, by Escher in 184L Still there can be no question that 

 Logan's 1860 letter to Barrande furnishes one of the main landmarks of 

 tectonic science. 



Almost as soon as Logan recognised the north-westward frontal 

 thrusting of the Caledonian Mountains of Canada he realized that it 

 followed a much older line of slope, leading down south-eastwards from 

 the platform of Laurentia to tne comparative depths of the Caledonian 

 sea bottom. He based this conception on the fact that the thrusts often 

 bring forward thick developments of fossiliferous Palaeozoic sediments 

 that are older than anything in the local unmoved Palaeozoic succession 

 of the over-ridden foreland. For instance, near Quebec the thrust-masses 

 include thick Lower Ordovician sediments, and very probably Cambrian 

 as well, whereas the unmoved Palaeozoic succession commences with Middle 

 Ordovician resting directly on Precambrian gneiss. 



Logan gave his theoretical slope a double function. First of all it 

 had to act as a boundary to early sedimentation, and then as a guide to 

 later thrusting and folding : — 



' The resistance offered by the buttress of gneiss,' said he, ' would not 

 only limit the main disturbance ; but it would probably also guide or 

 modify, in some degree, the whole series of parallel corrugations, and 

 thus act as one of the causes giving a direction to the great Appalachian 

 Chain of mountains.' 



There is, however, another aspect of Logan's Slope that has not, I 

 think, attracted sufficient attention. This slope, when completely sub- 

 merged, seems to have furnished a dividing line between clear-water 

 Ordovician limestones (American facies), that grew on its top to the north- 

 west, and muds and sands (Caledonian facies), that, creeping from the 

 opposite direction, came to rest at its foot. The fossils of the two sets 

 of deposits are as distinct as the rocks themselves, and this has led certain 

 distinguished palaeontologists to postulate continuous land barriers, or 

 isthmuses, separating the two fields of accumulation. On the other hand 

 I think it can be established that the limestone of the one field has 

 repeatedly landslipped down upon the mud of the other ; in which case 

 the division cannot have been an isthmus, but merely a submarine slope. 



The conception of the Logan Slope that I am now about to present is 

 a slight modification of Logan's original. Let us picture the slope, not 

 as a rigid feature of Precambrian date, eventually obliterated by 

 Palaeozoic sedimentation, but as tectonic in origin and intermittently 

 renewed by hinged subsidence. Earthquakes connected with the inter- 

 mittent renewal were probably responsible for the landslips to which I 

 have just alluded. It is well known that most of the major earthquakes 



