534 HISTORICAL GEOLOGY. 



are found in western England and Wales, where the strata claim a thickness 

 exceeding 20,000 feet independently of ash-ejections. The Upper Lland- 

 overy and other Upper Silurian beds rest iipon the upturned edges of the 

 Lower Llandovery, Caradoc, or other inferior strata. " In one district, 

 between the Longmynd and Wenlock Edge, the base of the Upper Silurian 

 rocks is found within a few miles to pass from the Caradoc group across to 

 the Lower Cainbrian rocks." (Geikie, p. 672.) 



Another renuxrkable region of disturbance is that of the northwest 

 Highlands of Scotland, along the chain of mountains between Eriboll and 

 Ullapool. For some distance east of this region, according to the investi- 

 gations of Hicks, Lapworth, Peach, Home, and others, the Silurian and 

 Cambrian rocks, which overlie the Archaean, are much plicated, and the 

 plications, on nearing it, become overthrust flexures, often flexure-faulted, 

 with the thrust westward. Then commences over the wide region a series 

 of nearly horizontal thrust-planes of great extent, along which the Archsean 

 and overlying formations are thrust westward, in some places for ten miles. 



Besides minor shovings, there are three maximum thrust-planes which 

 overlap so as to carry the formations over one another, pile them to a great 

 thickness, and produce a series of extensive unconformabilities between 

 Archaean, Cambrian, and Lower Silurian terranes ; and undisturbed Lower 

 Silurian limestone is often the base of the pile, with Archaean rocks above. 

 The thrust-planes look like planes of bedding, and were long so considered. 

 Under the enormous amount of friction along the lower thrust-plane, the 

 materials at the bottom of the moving mass were sometimes folded over and 

 curved under it as well as abraded or crushed ; and, in addition, through the 

 aid of the heat generated, sheets of sericite schist were made along the plane 

 out of the abraded feldspar, and layers of other foliated metamorphic rock 

 out of other material, — the strike of the foliation being more or less 

 parallel with that of the thrust-plane. 



In some cases the softer pebbles of a Cambrian conglomerate (made of 

 pebbles of quartz, gneiss, dioryte, granite) are drawn out so as to form 

 "thin lenticular bands of mica or hornblende schist flowing round the 

 harder pebbles of quartz-rock " ; and at one place Cambrian sandstones have 

 been converted into schists containing mica, and quartzytes merge into 

 quartzose sericite schists. The fossiliferous Silurian limestones below the 

 thrust-plane are not generally altered, but in some places have been ren- 

 dered crystalline. {Q. J. G. Soc, 1884, 1888, the latter giving full references 

 to earlier writers on the subject.) 



In northern Ireland, where the Lower Silurian and Cambrian beds have 

 a thickness of more than 7000 feet, there are evidences of metamorphism in 

 portions of the beds, while others still retain their fossils, and mark their 

 Siluro-Cambrian age. The Upper Silurian beds above are undisturbed and 

 unaltered. Geikie states that the crystalline schists of the Scottish High- 

 lands are prolonged over northern Ireland to Galway Bay, which makes the 

 disturbed region 400 miles long. 



