PART I. CHAPTER IV. 51 



Consolidation of Strata. 



on these reefs, are very commonly found to be encrusted over 

 with a hard coating of limestone.* 



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 

 horizontally of stratd are strictly applicable to mechanical depo- 

 sits, 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 encrust the vertical walls of a fissure, and be of equal 

 thickness throughout. ; but such deposits are of small extent, and 

 for the most part confined to vein-stones. 



Cementing of particles. It is chiefly in the case of calca- 

 reous rocks that solidification takes place at the time of deposi- 

 tion. But there are many deposits in which a cementing process 

 comes into operation long subsequently. We may sometimes 

 observe, where the water of ferruginous or calcareous springs 

 has flowed through a bed of sand or gravel, that iron or carbo- 

 nate 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, belong- 

 ing 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 stony near Kelloway. In this 

 district there are numerous fossil shells which have decomposed, 

 having for the most part left only their casts. The calcareous 

 matter hence derived has evidently served, at some former period, 

 as a cement to the siliceous grains of sand, and thus a solid 

 sandstone has been produced. If we take fragments of many 

 other argillaceous grits, retaining the casts of shells, and plunge 

 them into dilute muriatic or other acid, we see them immediately 

 changed into common sand and mud; the cement of lime, 

 derived from the shells, having been dissolved by the acid. 



Traces of impressions and casts are often extremely faint. In 

 some loose sands of recent date we meet with shells in so 

 advanced a stage of decomposition as to crumble into powder 

 when touched. It is clear that water percolating such strata may 

 soon remove the calcareous matter of the shell ; and, unless 

 circumstances cause the carbonate of lime to be again deposited, 



* See Principles of Geology, Index, "Travertin," "Coral reefs," &c. 



