A. J. Jukes-Browne — Oamhrian Geography. 503 



whole the lowest proportion of insoluble residue was obtained from 

 the Brachiopod limestones and the Oolitic bands. 



My examination therefore confirms the view expressed by 

 Professor Skeats ("Dolomites of the Southern Tyrol," Q.J.G.S., 

 1905), viz., that chemically pure limestones are formed by rapid 

 deposition near a coastline which is usually composed of calcareous 

 rocks, from the waste of which the material is obtained. 



On the other hand, I have obtained no evidence that such limestones 

 are formed necessarily under coral-reef conditions, but rather that 

 such conditions are by no means essential, and that the amount of 

 insoluble residue depends on the action of the currents. 



The analyses in this paper were carried out in the Geological 

 Laboratories of the Imperial College of Science and Technology, and 

 I wish to express my gratitude for much kind advice and assistance to 

 Professor W. W. Watts and other members of the staff of the Collecre. 



Y. — Cambrian Geography. 



By A. J. Jukes-Browne, F.E.S., F.G.S. 



PROFESSOR PEACH'S address, as President of Section C at the 

 recent meeting of the British Association, concludes with some 

 interesting remarks on the geographical conditions of Cambrian time, 

 and these may be regarded as an amplification of the views which he 

 had previously expressed to the Koyal Society of Edinburgh (1886) 

 and in the Geological Survey memoir on the Structure of the 

 N. W. Highlands of Scotland ( 1 907). 



I had occasion to comment on these views in the recent edition of 

 my Building of the British Isles (1911), and am glad to find I correctly 

 interpreted his original suggestion as meaning that a sea with 

 continuous shore-lines stretched across the Atlantic region from 

 Canada to Ireland, and that this sea separated two different zoological 

 provinces of littoral Invertebrata. 



In this general statement there is now a complete agreement 

 between us, because, as he says, it seems a reasonable inference from 

 the paleeontological evidence, but it is not so easy to follow the 

 arguments by which he endeavours to support the more detailed 

 exposition of his hypothesis. 



He begins by assuming that the materials of the Torridon Sand- 

 stone and of the newer Algonkian rocks of Canada were derived from 

 a land which formed a continuous belt from Scandinavia to North 

 America. He continues : " During the period which elapsed between 

 the deposition of the Torridon Sandstone and the basement members 

 of the Cambrian system a geosyncline was established which gave 

 rise to a submarine trough, trending in an E.N.E. and W.S.W. 

 direction both in the British and North American areas. In the 

 latter region it extends from Newfoundland to Alabama, its south- 

 eastern limit being defined by the old land surface of Appalachia. 

 The extension of this Appalachian land area in a north-east direction, 

 beyond the limits of Nova Scotia and Newfoundland, was postulated 

 by Dana and other American writers." 



