DIFFUSION OF THE CAMBRIAN FAUNAS. 7 



pressure arising from the continual subsidence of the crust of the earth under the great 

 ocean troughs, which have formed along the borders of the continents. Dana teaches that 

 these troughs or abysses in the Atlantic Ocean originated before the Cambrian age, but 

 have continued to increase in depth as the ages rolled on. This view has been contested 

 by other writers, but it certainly seems to accord with what we know of the distribiition 

 of the Cambrian faunas. 



It is in accordance with this supposed ridging up of the continental borders that wo 

 find the ITpper Cambrian faunas every where poorly represented in eastern North America 

 (except along the Atlantic coast) ; and that we find the deep-water phase of the Lower 

 Cambrian faunas absent from this region, or nearly so. In the Upper St. Lawrence region 

 (including L. Champlain) scarcely a vestige exists of the rich faunas which in Europe 

 lie between the Oleuellus horizon and the Ordovician. Only the meagre fauna of the 

 typical Potsdam, and the shred of measures containing Dicellocephali which overlie them 

 at one locality, are present, to suggest the riches of the organic world elsewhere. And in 

 the same manner the blank between the Olenellus beds in the north of Scotland and the 

 Durness Limestone, exactly reproduces the conditions i^revalent in the north of the Gulf 

 of St. Lawrence, where Lingula acuminata is the only species linking the Lower Cam- 

 brian Olenellus beds with those containing a fauna almost Ordovician in its faciès.' In 

 Russia this hiatus has been partly filled by the introduction of the Dictyouema beds ; but 

 it is only in Scandinavia. "Wales and on the Atlantic coast in British America that the 

 intermediate faunas are fully represented. The inference is natural that the uniformity 

 in this respect was due to similar conditions of temperature and depth continued in these 

 regions throughout Cambrian time. 



Reverting- to the western side of the Laurentian nucleus, this on the contrary appears 

 to have been laved by the waters of a shallow ocean. The off-shore deposits here are 

 concealed by more recent formations, but those immediately bordering the metamorphic 

 rocks of the nucleus are shallow water limestones, sandstones, and these by degrees give 

 place to the widely diffused limestones of the Trenton period. 



Thus the known conditions point to shallow seas with warm water on the western 

 side of the nucleus, and deep seas with cold water on the eastern side. There were, how- 

 ever, on this side two submerged plateaux, covered by a shallow warm sea throughout 

 the Cambrian age, viz., one at the base of the Adirondack Mountains, and the other far to 

 the uorth-east, near the Strait of Belle Isle. 



IV. — Opinions of Professor Jules Mabcou and C. D. "Walcott. 



Several writers have attempted to explain the peculiar distribution of the Cambrian 

 faunas on grounds more or less diverse from those above stated. One of them. Professor 

 Jules Marcou in a series of papers pixblished in the American Geologist, which have 

 attracted much attention, explains the contrast of these faunas. He supposes that 

 in Cambrian times there were two seas in the north temperate region of the western 

 hemisphere, each inhabited by its own fauna. These seas he says were divided from 

 each other by a narrow isthmus. One of the seas extended over the middle and southern 



'See Palseozoic Fossils, E. Billings, vol. I., p. 371. 



