24 



SLOW DEPOSITION OF STRATA. 



[Ch. in. 



manner, a piece of fossil wood (a, fig. 14) has been perforated by an 

 animal of a kindred but extinct genus, called Teredina by Lamarck. 

 The calcareous tube of this mollusk was united and as it were soldered 



Fig. 14. 



Fossil and recent wood drilled by perforating Mollusca. 

 Fig. 14. a. Fossil wood from London clay, bored by Teredina. 



o. Shell and tube of Teredina personate, the right-hand figure the ventral, the left the 

 dorsal view. 

 Fig. 15. e. Eecent wood bored by Teredo. 



d. Shell and tube of Teredo navalis, from the same. 



o. Anterior and posterior view of the valves of same detached from the tube. 



on to the valves of the shell (b), which therefore cannot be detached 

 from the tube, like the valves of the recent Teredo. The wood in this 

 fossil specimen is now converted into a stony mass, a mixture of clay 

 and lime ; but it must once have been buoyant and floating in the sea, 

 when the Teredinoe lived upon it, perforating it in all directions. Again, 

 before the infant colony settled upon the drift-wood, the branch of a tree 

 must have been floated down to the sea by a river, uprooted, perhaps, by 

 a flood, or torn off and cast into the waves by the wind : and thus our 

 thoughts are carried back to a prior period, when the tree grew for years 

 on dry land, enjoying a fit soil and climate. 



It has been already remarked that there are rocks in the interior of 

 continents, at various depths in the earth, and at great heights above the 

 sea, almost entirely made up of the remains of zoophytes and testacea. 

 Such masses may be compared to modern oyster-beds and coral reefs; 

 and, like them, the rate of increase must have been extremely gradual. 

 But there are a variety of stony deposits in the earth's crust, now proved 

 to have been derived from plants and animals, of which the organic ori- 

 gin was not suspected until of late years, even by naturalists. Great 

 surprise was therefore created by the recent discovery of Professor Ehren- 

 berg of Berlin, that a certain kind of siliceous stone, called tripoli, was 

 entirely composed of millions of the remains of organic beings, which 

 the Prussian naturalist refers to microscopic Infusoria, but which most 

 others now believe to be plants. They abound in freshwater lakes and 

 ponds in England and other countries, and are termed Diatomaceas by 

 those naturalists who believe in their vegetable origin. The substance 



