IGNEOUS ROCKS 127 



disintegration if its cement is redissolved. The process is analogous 

 to the consolidation of road mud by frost, which converts water 

 between the dust grains into crystalline ice. The road surface 

 behaves as a hard rock so long as it remains frozen. Sandstones, 

 ironstones, and sandy limestones illustrate this process, and the 

 cement can often be removed by hydrochloric acid or aqua regia, 

 the rock being thus reduced to its original condition of loose 

 sediment. Some of the cements, particularly salts of iron and 

 manganese, are responsible for the coloration of rocks. Modern 

 calcareous deposits, such as shell beds and coral rock, sometimes 

 become very thoroughly cemented and compacted by the solution 

 of part of their carbonate of lime and the deposit of it between the 

 fragments and in the interstices of the organisms. This chemical 

 rearrangement is of common occurrence, and is responsible 

 sometimes for hardening the whole rock, sometimes for solidifying 

 only certain bands or nodules in it. These latter are known as 

 concretionary bands or nodules. A striking example is the flint 

 nodules in Chalk, which are only portions of the chalk 

 replaced by silica deposited in isolated spots in the chalky 

 limestone from solution. Such nodules are practically pure 

 silica, and they are often found to contain organisms like sponges, 

 which originally had siliceous skeletons, and also fossil shells and 

 sea urchins, which when living consisted of carbonate of lime but 

 are now replaced by silica without loss of their original shapes and 

 structures. 



Other effects of elevation and the forces causing it are 

 mechanical, and there may be mentioned cleavage, jointing, 

 and faulting. Cleavage is a tendency which certain fine- 

 grained rocks possess to split into thin layers, called slates, along 

 planes not usually coincident with the original stratification. 

 This is the outcome of further application of the same lateral 

 pressure which began by folding and hardening the rocks, but 

 was so intense later that the rock constituents were compelled to 

 set themselves at right angles to the direction of pressure. In 

 consequence of this a new grain was given to the rock, which now 

 splits parallel to the up-ended particles into thin elastic slabs 

 which are of great value for roofing purposes. 



Joints are fissures, more or less open, along which the rocks 



