24 THE CHAIN OF LIFE. 



(Fig. 14). The former may be conveniently divided into three 

 members : First, the Bojian, or Ottawa gneiss, consisting of 

 stratified granite rocks, usually of a red colour, and of very 

 great thickness. This contains, so far as known, no limestone, 

 and has afforded as yet no trace of fossils. Secondly, the 

 Middle Laurentian, the greater part of which consists of gneiss, 

 but containing important beds of other rocks, as quartzite, iron 

 ore, and limestone. It is in this series that we have the first 

 evidence of life, and it is here also that we find the greatest 

 abundance of carbon, in ^he form of graphite or plumbago, and 

 also of calcium phosphate, or bone earth. Thirdly, the Upper 

 Laurentian or Norian series. This consists in great part of 

 Labadorite, or lime feldspar, but has also beds of ordinary 

 gneiss, limestone, and iron ore. 



T!.Hl|iAJ.I.|!)JJ, gcrrii e 



a b c d 



Fig. 14. — Ideal section, showing the relations of the Laurentian and Huronian. 



a. Lower Laurentian. h, Middle Laurentiarr. c. Upper Laurentian. d, Huronian. 



e, Cambrian and Silurian. 



The latter, the Huronian, is much less crystalline, and is 

 divisible into two series — the Lower Huronian, which includes 

 many beds of volcanic origin, and the Upper Huronian, which 

 has afforded some obscure fossils. The Huronian was first 

 recognised by Sir W. E. Logan in Canada, but corresponding 

 rocks exist in Europe. The Pebidian series of Hicks in Wales 

 is probably of this age. 



It is likely that much of the present appearance and con- 

 dition of the most ancient rocks may be attributed to meta- 

 morphism, that is, to the slow baking under the influence of 

 heat, heated water, and pressure, to which they have been 

 subjected in the lower parts of the earth's crust, when buried 

 deeply under newer deposits. It is also true, however, as 



iifii 



