8 INTRODUCTION. 



of boring-animals is very great ; besides which numerous 

 fishes subsist by browsing on the Hving branches of coral. ^ 



Again, the siliceous matter in beds of chert and in nodules 

 of flint may be due in the first instances to the accumulation 

 of organisms having siliceous structures, but the formation of 

 the beds and nodules themselves is evidently due to inorganic 

 agency. Both flint and chert are formed in rocks of marine 

 and freshwater origin, by the indirect agency of plants and 

 animals. 



Deposits of very varied origin may be commingled in the 

 neighbourhood of active Volcanoes, the ashes from which are 

 frequently carried to great distances, and may be deposited 

 on the ocean-bed as a sedimentary deposit, together with 

 organic and other accumulations. 



To turn to the subject of Igneous rocks, we find that 

 although a study of the volcanic phenomena that are presented 

 to our view at the present day throws considerable light on 

 their former history, yet our opportunities of observation are 

 necessarily limited to the rocks now thrown up at the surface. 

 On the other hand, many of the old igneous rocks, belonging 

 to the various geological epochs, have evidently been formed 

 or intruded at a depth below the surface, and have perhaps 

 never appeared in the light of day, until comparatively recent 

 times, when disturbances and denudation have together as- 

 sisted to reveal them. The volcanic action of the present day, 

 however, that may be witnessed in different parts of the world, 

 enables us to explain the origin of most of our igneous rocks, 

 and to demonstrate the former presence in Britain of centres 

 of eruption. 



Igneous rocks are of all ages, and occur as bosses and 

 dykes bursting through and penetrating the stratified rocks. 

 Sometimes they have flowed over a surface upon which other 

 beds were afterwards deposited, so that they are intercalated 

 with these rocks ; at other times they have forced their way 

 between already hardened beds, and given a false appearance 

 of contemporaneity. 



Rocks, too, which were once deposited under water, have 

 in part or entirely lost their original stratified character, and 

 become altered or metamorphosed. This may result from 

 great mechanical disturbance, and from molecular or purely 

 chemical changes, due to contact with igneous rocks, and 

 the influence of heated waters, and attended perhaps by 

 the loss or addition of certain elements. Igneous rocks 

 may also be metamorphosed. All rocks, indeed, are in 



1 Proc. G. S. ii. 576. - 



