Dr. H. Hicks — Life Zones in Pal(BOZoio Rocks. 403 



the Northern Hemisphere. In this paper, page 157, I stated that 

 "on both sides of the Atlantic they (the pre-Cambrian rocks) are to 

 be seen at various places from the latitude of 30° to the Arctic 

 regions, and it seems reasonable to suppose that these portions 

 indicate parts only of what were probably two great continents 

 extending over these areas, separated from one another by an 

 intermediate ocean, narrower considerably than the present Atlantic, 

 but still occupying part of that basin. When these continents 

 commenced to subside, the parts facing the Atlantic were the first 

 to become submerged." ^ 



The great thicknesses found in some areas show that there must 

 have been much loose material on the pre-Cambrian land ready to 

 be washed off as each part became depressed, and an examination of 

 the sediments shows very clearly that much of the material came 

 from the volcanic rocks and cones of the pi-e-Cambrian period.- 

 Angular fragments of volcanic rocks occur so frequently, mixed up 

 with other sediments in the Lower Cambrian rocks, that it seems 

 clear that such loose materials remained on the higher ridges and 

 plateaux after adjoining areas had been depressed to a considerable • 

 depth ; and the incoming of these materials along with other rough 

 sediments, usually at fairly definite horizons, mark frequently the 

 vertical limits of certain organisms in these areas. 



At the meeting of the International Geological Congress in London 

 in 1888, Mr. C. Walcott, of the United States Geological Survey, 

 made the very important announcement that he had recently dis- 

 covered in a very complete section of the Cambrian rocks at Manuel's 

 Brook, Newfoundland, that the Olenellus fauna occurred below the 

 Paradoxides fauna, and not above, as had up to that time been 

 supposed to be the case in America. In Sweden and in some other 

 areas in the N. of Europe it had been previously shown by Doctors 

 Linnarsson, Brogger, Holm, and Schmidt that the Olenellus fauna 

 was the oldest ; but until Mr. Walcott showed that the order of 

 succession was similar in America, it had not been taken as of 

 general application. Mr. Walcott now proposed also that the 

 Cambrian should be divided into three parts, viz. the Loioer, 

 characterized by the Olenellus, the Middle by the Paradoxides, and 

 the Upper by the Olenus fauna, and this classification has since 

 been very generally adopted. Previously in Wales we had divided 

 the Cambrian into two main parts, viz. a Lower containing the 

 Paradoxides and earlier faunas, and an Upper the Olenus fauna. 

 I had, however, as already shown, subdivided in 1881 the then 

 Lower Cambrian into three groups, viz. Caerfai, Solva, and Menevian ; 

 and in the year 1892 ^ I was able to show that the Olenellus fauna at 

 St. David's was confined to the Caerfai Group. Here the Paradoxides 



^ Mr. C. Walcott has recently worked out with much care the evidences of the 

 early conditions prevailing during the same periods on the American continent 

 ("The Fauna of the Lower Cambrian or Olenellus Zone," 1890, and "Bulletin of 

 the United States Geological Survey," No. 81, 1891). 



^ See " Pre-Cambrian Volcanoes and Glaciers," Geol. Mag. Dec. II. Vol. YII. 

 (November, 1880). 



3 Geol. Mag. Dec. III. Vol. IX. p. 22. 



