INTRODUCTION. Vll 



would afford the necessary shelter or food to certain 

 animals. Indeed the continual motion of the stream 

 and the destructive property of fresh water might pre- 

 clude the possibility of any marine animals existing 

 within the prohibited area ; and in that case the central 

 bed of the channel would be partially covered with clear 

 sand, devoid of organic remains. An instance of this 

 kind has been given by Dr. Wallich in his account of 

 Hamilton's Inlet, Labrador *. Such cases, however, are 

 exceptional ; and the limits of these areas are very cir- 

 cumscribed. Many kinds of Invertebrata are known to 

 nourish in the most rapid tideways, and even in whirl- 

 pools; and the water of the ocean everywhere teems 

 with life. The dredge has never failed to bring up 

 some organisms from every part of the sea-bed which 

 has been hitherto explored. However unpromising it 

 may at first sight appear, the cleanest-looking sand 

 taken from any depth of water, and carefully examined 

 by the aid of a lens or microscope, will be found to con- 

 tain structural forms. Having these facts and a certain 

 degree of experience to guide us in the inquiry, it would 

 be a hasty assumption that any geological strata of 

 comparatively recent formation, which do not contain 

 fossils, are of marine origin. Whether the absence of 

 fossils from particular strata may be attributable to 

 chemical absorption or decomposition is a question 

 which must be decided on other grounds. Let Mr. 

 Sorby be the exponent. Until more is known of this 

 difficult and interesting subject, we may suspend our 

 judgment as to any formation being either azoic or pro- 

 tozoic. The presence of scarcely more distinct traces of 

 life than a few worm-casts in our lowest Silurian rocks 

 does not prove the improbability, much less the impos- 

 * ' The North-Atlantic Sea-bed,' p. 49. 



