SILURIAN. 251 



granite and which contain fossils of late Silurian and early Devonian age, as deter- 

 mined by Sir William Dawson, by Logan, and on fuller collections by Ami. 



Farther northeast in Pictou and Colchester counties, Fletcher ^^^^ distinguishes 

 strata which he assigns to Medina, Clinton, and Niagara horizons. Fossils deter- 

 mined by Ami include Cyrtograptus grayice, " characteristic of the lower half of the 

 Silurian system." 



The Silurian rocks of Arisaig, in Antigonish County, on the coast of Northum- 

 berland Strait, were first described by Honeyman.*" Fletcher ^^^ gives a somewhat 

 detailed section, from which the following is condensed : 



Feet. 



liower Helderberg, argillaceous sandy flags with thin fossiliferous limestone 1,038 



Niagara, argillaceous, calcareous rocks with numerous layers of richly fossiliferous limestone 



(400 feet) followed by dark shales, also fossiliferous 1,293 



Upper Clinton, green arenaceous shales and fine sandstone 148 



(Break in section.) 



Lower Clinton, dark-gray, crumbly, papery argillites 345 



Medina, greenish to gray, more or less massive sandstone 182 



These strata lie in a "syncline, 6 miles long and IJ wide, bounded on the south 

 by a fault indicated by an escarpment of Cambro-Silurian rocks; on the north by 

 pre-Cambrian; and on the west by Devonian and Carboniferous, which overlie 

 them." The relation of conformity or unconformity to the "Cambro-Silurian" 

 (Ordovician) is not clearly stated. With reference to the Silurian-Devonian con- 

 tact Ami ^** suggests a possible unconformity, but says: " On the whole, the general 

 trend and behavior of the strata referred to the Silurian and Devonian systems are 

 fairly uniform and generally identical." On the other hand, the lower Carboniferous 

 conglomerates and limestone extend with marked unconformity across the Silurian 

 and Devonian. 



The Arisaig section and other sections of the Silurian in Nova Scotia have 

 been classified by Ami " as follows : 



The Silurian system as understood in Canada, and restricted to the upper division of Sir 

 Roderick Murchison's Silurian, is extensively developed both in Nova Scotia and New Bruns- 

 wick. At Arisaig, in Antigonish County, Nova Scotia, several thousand feet of more or less 

 disturbed and inclined strata, including an almost regular succession of different members of 

 this system, made up of sandstones, slates, iron ores, and black graptolitic slates and limestones 

 with mudstones, are well exposed and present a compact fauna, which in facies closely resembles 

 rocks in Herefordshire, in Cumberland, Westmoreland, in the Kendal and Ludlow regions of 

 England. The "Knoydart" formation, consisting of red shales and sandstones and calcareous 

 bands holding pteraspidian and ostracoderm fishes and crustaceans referable to the Comstone 

 or lower Old Red sandstone of Great Britain, almost immediately overlies the SUurian strata 

 though no actual contact has been observed. The SUurian series at Arisaig consists of at least 

 four distiuct geological formations. Beginniug above we have first the "Stonehouse" forma- 

 tion, consisting for the most part of dark-red, fine-grained shales and mudstones, holding a 

 conspicuous lamellibranchiate fauna, of which Grammysia acadica Billings is a weU-known 

 species, together with a number of interstratified more or less thin calcareous bands holding 

 brachiopods, gastropods, trilobites, and ostracods in abundance. Below this we find the 

 "Moydart" formation, which consists of more or less heavy bedded light greenish-gray and 

 rusty-weathering calcareous strata (in which the "Red stratum" of authors occurs) and holds 

 brachiopods, gastropods, cephalopods, and crinoids. Beneath this again we have the ' ' McAdam " 

 formation, consisting for the most part of impure black carbonaceous shales, which are spHntery 



