34 CONSOLIDATION OF STRATA. [Ch. IT. 



geologists between deposits of a chemical, and those of a mechanical, 

 origin. By the latter name are designated beds of mud, sand, or peb- 

 bles produced by the action of running water, also accumulations of 

 stones and scoriae thrown out by a volcano, which have fallen into their 

 present place by the force of gravitation. But the matter which forms 

 a chemical deposit has not been mechanically suspended in water, but in 

 a state of solution until separated by chemical action. In this manner 

 carbonate of lime is often precipitated upon the bottom of lakes and 

 seas in a solid form, as may be well seen in many parts of Italy, where 

 mineral springs abound, and where the calcareous stone, called travertin, 

 is deposited. In these springs the lime is usually held in solution by an 

 excess of carbonic acid, or by heat if it be a hot spring, until the water, 

 on issuing from the earth, cools or loses part of its acid. The calcareous 

 matter then falls down in a solid, state, incrusting shells, fragments of 

 wood and leaves, and binding them together.* 



In coral reefs, large masses of limestone are formed by the stony skel- 

 etons of zoophytes ; and these, together with shells, become cemented 

 together by carbonate of lime, part of which is probably furnished to 

 the sea-water by the decomposition of dead corals. Even shells of which 

 the animals are still living, on these reefs, are very commonly found to 

 be incrusted over with a hard coating of limestone.f 



If sand and pebbles are carried by a river into the sea, and these 

 are bound together immediately by carbonate of lime, the deposit 

 may be described as of a mixed origin, partly chemical, and partly 

 mechanical. 



Now, the remarks already made in Chapter II. on the original hori- 

 zontality of strata are strictly applicable to mechanical deposits, and 

 only partially to those of a mixed nature. Such as are purely chemical 

 may be formed on a very steep slope, or may even incrust the vertical 

 walls of a fissure, and be of equal thickness throughout ; but such de- 

 posits are of small extent, and for the most part confined to vein-stones. 



Cementing of particles. — It is chiefly in the case of calcareous rocks 

 that solidification takes place at the time of deposition. But there are 

 many deposits in which a cementing process comes into operation long 

 afterwards. "We may sometimes observe, where the water of ferruginous 

 or calcareous springs has flowed through a bed of sand or gravel, that 

 iron or carbonate of lime has been deposited in the interstices between 

 the grains or pebbles, so that in certain places the whole has been bound 

 together into a stone, the same set of strata remaining in other parts 

 loose and incoherent. 



Proofs of a similar cementing action are seen in a rock at Kelloway 

 in Wiltshire. A peculiar band of sandy strata, belonging to the group 

 called Oolite by geologists, may be traced through several counties, the 

 sand being for the most part loose and unconsolidated, but becoming 



* See Principles, Index, " Calcareous Springs," (fee. 

 f Ibid. « Travertin," " Coral Reefs," &c. 



