50 THE STOEY OF THE EAETH AND MAN. 



selected with that view, it would be quite easy to 

 suppose that those lengthened by distortion are of 

 different species from those distorted so as to be 

 shortened. Slaty cleavage and distortion are not, 

 however, confined to Primordial rocks, but occur in 

 altered sediments of various ages. 



The Primordial sediments must have at one time 

 been very widely distributed, and must have filled up 

 many of the inequalities produced by the rending and 

 contortion of the Laurentian beds. Their thicker and 

 more massive portions are, however, necessarily along 

 the borders of the Laurentian continents, and as they 

 in their turn were raised up into land, they became 

 exposed to the denuding action first of the sea, and 

 afterwards of the rain and rivers, and were so exten- 

 sively wasted away that only in a few regions do large 

 areas of them remain visible. That of Bohemia has 

 afforded to Barrande a great number of most interest- 

 ing fossils. The rocks of St. David's in Wales, thoso 

 of Shropshire in England, and those of Wicklow in 

 Ireland are also of great interest ; and next to these 

 in importance are, perhaps, the Huronian and Acadian 

 groups of North America, in which continent as for 

 example in Nova Scotia and in some parts of New 

 England there are extensive areas of old metamor- 

 phic rocks whose age has not been determined by 

 fossils, but which may belong to this period. 



The question of division lines of formations is one 

 much agitated in the case of the Cambrian rocks. 

 Whether certain beds are to be called Cambrian or 



