Xll GEOLOGICAL FEATURES 



All rivers and streams contain a certain quantity of car- 

 bonic acid gas which they derive from the atmosphere ; and 

 so, wherever they come in contact with carbonate of lime, 

 which is one of the most common substances in the crust of 

 the earth, they dissolve some portion of it, which is carried 

 into the sea in a state of solution. The sea contains five 

 times as much carbonic acid gas as is sufficient to keep the 

 lime which is brought into it in solution, so that it cannot be 

 deposited at the bottom, but is secreted from the ocean for 

 food and building-material by coral insects, or by other more 

 minute microscopic animalcules which live near the surface, 

 and whose shells are found in countless numbers at the 

 bottom of the ocean*. Microscopic research within the last 

 ten years has made great additions to this, as well as to 

 other branches of science. From soundings taken by 

 Brooke's Deep- Sea Sounding Apparatus at the depth of 

 more than two miles, on the telegraphic plateau in the middle 

 of the Atlantic, the bottom was found to be almost entirely 

 made up of little calcareous shells (Foraminifera) which 

 can only be detected by the microscope. The remainder of 

 the bottom was found to consist of siliceous or flinty skeletons 

 of minute animal (Infusoria) and vegetable bodies (Diato- 

 maceae). 



In other parts of the sea the bottom has been found to 

 be formed almost entirely of siliceous or flinty microscopic 

 bodies, and they abound in all seas. So small are these 

 creatures and so abundant, that although it takes a hundred 

 millions of them to weigh a grain, Professor Ehrenberg 

 states that in the harbour of Wismar, in the Baltic, nearly 

 18,000 cubic feet of them are deposited in a year ; and they 

 are so prolific that " a single one of them can increase to 



* These animals are provided with a ciliary or hairy apparatus, by 

 the vibration of which they cause currents of water to flow into their 

 mouths ; they then secrete or separate the lime which is held in solu- 

 tion, and reject the water. 



