52 THE STOEY OF THE EARTH AND MAN. 



the Lower Silurian were deposited ; and in some cases 

 the latter rest on weather-worn and pitted surfaces, 

 and are filled with angular bits of the underlying rock, 

 as well as with drift-shells which have been cast on 

 these old Laurentian shores ; while in other cases the 

 Silurian rests on smooth water-worn Laurentian rocks, 

 and is filled at the junction with well-rounded pebbles 

 and grains of sand which have evidently been subjected 

 to a more thorough attrition than those of the present 

 beach. With respect to the line of division between 

 the Primordial and the next succeeding rocks, it will 

 be seen that important movements of the continents 

 occurred at the close of the Cambrian, and in some 

 places the Cambrian rocks have been much disturbed 

 before the deposition of the Lower Silurian. 



Seated on some ancient promontory of the Lau- 

 rentian, and looking over the plain which, in the 

 Primordial and Lower Silurian periods was the sea, I 

 have often wished for some shred of vegetable matter 

 to tell what lived on that land when the Primordial 

 surf beat upon its shore, and washed up the Trilobites 

 and Brachiopods of those old seas ; but no rock has 

 yet taken up its parable to reveal the secret, and the 

 Primordial is vocal only with the old story : " And God 

 said, Let the waters swarm with swarming living 

 things, and it was so." So our picture of the period 

 may represent a sea-bottom swarming with animals of 

 low grade, some sessile, some locomotive ; and we may 

 merely suppose a distant shore with vegetation dimly 

 seen, and active volcanoes ; but a shore on which no 



