THE MODEIiX PKItlOD. 17 



wliicli, should tins subsidence go on, will creep slowly back to 

 reoccupy the ground which it once held in the Post-pliocene time. 



Such peculiarities of distribution serve to show the effects of even 

 comparatively small changes of level upon climate, and upon the 

 distribution of life, and to confirm the same lesson of caution in our 

 interpretation of local diversities of fossils, which geologists have 

 been lately learning from the distribution of cold and warm currents 

 in the Atlantic. Another lesson which they teach is the wonderful 

 fixity of species. Continents rise and siidc, climates change, islands 

 are devoured by the sea or restored again from its dei)tlis, marine 

 animals are locally exterminated, and are enabled in the course of 

 long ages to regain their lost abodes, yet they remain ever the same, 

 and even in their varietal forms perfectly resemble those remote 

 ancestors which are separated from them by a vast lapse of ages and 

 by many physical revolutions. This truth, which I have already 

 deduced from the Post-pliocene fauna of the St Lawrence Valley, is 

 equally taught by the molluscs of the Acadian Piay, and by their 

 Arctic relatives returning after long absence to claim their old 

 homes. 



Oyster-leds or Mussel-beds. — In the bays and estuaries of Prince 

 Edward Island, and of the northern coast of Nova Scotia, vast ac- 

 cumulations of the shells of the American oyster, Ostrea Virginiana, 

 and those of the mussel, Mytilus tdidis, have taken place, and must be 

 deposits of the modern period succeeding the Post-pliocene. I have 

 been informed by Mr W. H. Pope, who has given much attention to 

 this subject, that some of these beds are fifteen feet or more in thick- 

 ness. They consist of dead shells, and in many places no living 

 shells occur even at the surface, the animals having been killed by 

 the gradual approach of the beds to the surface of the water, exposing 

 them to the action of the frost and ice and to invasion of sandy sedi- 

 ment. These beds of dead oyster and mussel shells, with the mud 

 filling the interstices, constitute one of the most valuable deposits on 

 the island. Under the name of " Mussel Mud " this material is taken 

 up in great quantity by ingenious dredging machines, worked iVum 

 rafts in summer or from the ice in winter, and is applied as a manure 

 to the soil with the most excellent effects. It supplies lime and 

 organic matter, besides small quantities of phosphates and alkalies. 



The shells in these old beds arc all of the long narrow form (0. 

 Vrrginiana), and Mr Pope informs me that the round form (0. bore- 

 ales) occurs at the surface in many places wln-re the lung narrow 

 form is found only a few inches below. It also appears that the 

 modern oysters procured in the up[)cr parts of the rivers and on 



