10 INTRODUCTION. 



chief varieties, their mode of origin, and their historical succession. 

 It should be borne in mind, however, that there are cases in which 

 strictly volcanic deposits may come to contain the remains of animals 

 and plants. Thus, animals and plants may be enveloped and en- 

 tombed in showers of volcanic ashes falling upon land, and deposits 

 of subaerial volcanic ash may thus become fossiliferous. Moreover, 

 it is very common for volcanic ashes to fall in vast quantities into 

 the sea or into a lake, when they become subjected to the action of 

 water, and may envelop the animals living at the bottom. Hence 

 it is by no means unusual to meet in the crust of the earth with 

 more or less extensive deposits of volcanic ashes, which though 

 igneous in origin are secondarily aqueous, being not only stratified 

 but also containing the remains of aquatic animals. 



