S6 CONSOLIDATION" OF STRATA. [Ch. IY. 



dried, may afterwards be immersed for any lengtli of time in water 

 without becoming soft again. Hence it is found desirable to shape the 

 stones which are to be used in architecture while they are yet soft and 

 wet, and while they contain their " quarry-water," as it is called ; also to 

 break up stone intended for roads when soft, and then leave it to dry in 

 the air for months that it may harden. Such induration may perhaps 

 be accounted for by supposing the water, which penetrates the minutest 

 pores of rocks, to deposit, on evaporation, carbonate of lime, iron, silex, 

 and other minerals previously held in solution, and thereby to fill up the 

 pores partially. These particles, on crystallizing, would not only be 

 themselves deprived of freedom of motion, but would also bind together 

 other portions of the rock which before were loosely aggregated. On 

 the same principle wet sand and mud become as hard as stone when 

 frozen ; because one ingredient of the mass, namely, the water, has crys- 

 tallized, so as to hold firmly together all the separate particles of which 

 the loose mud and sand were composed. 



Dr. MacCulloch mentions a sandstone in Skye, which may be moulded 

 like dough when first found ; and some simple minerals, which are rigid 

 and as hard as glass in our cabinets, are often flexible and soft in their 

 native beds ; this is the case with asbestos, sahlite, tremolite, and 

 chalcedony, and it is reported also to happen in the case of the 

 beryl."-^ 



The marl recently deposited at the bottom of Lake Superior, in North 

 America, is soft, and often filled with freshwater shells ; but if a piece 

 be taken up and dried, it becomes so hard that it can only be broken by 

 a smart blow of the hammer. If the lake therefore was drained, such 

 a deposit would be found to consist of strata of marlstone, like that 

 observed in many ancient European formations, and like them contain- 

 ing freshwater shells. 



It is probable that some of the heterogeneous materials which rivers 

 transport to the sea may at once set under water, like the artificial mix- 

 ture called pozzolana, which consists of fine volcanic sand charged with 

 about 20 per cent, of oxide of iron, and the addition of a small quantity 

 of lime. This substance hardens, and becomes a solid stone in water, 

 and was used by the Romans in constructing the foundations of build- 

 ings in the sea. 



Consolidation in these cases is brought about by the action of chemical 

 affinity on finely comminuted matter previously suspended in water. 

 After deposition similar particles seem to exert a mutual attraction on 

 each other, and congi-egate together in particular spots, forming lumps, 

 nodules, and concretions. Thus in many argillaceous deposits there are 

 calcareous balls, or spherical concretions, ranged in layers parallel to the 

 general stratification ; an arrangement which took place after the shale 

 or marl had been thrown down in successive laminae ; for these laminse 



* Dr. MacCulloch, Syst. of Geol. vol. i. p. 123. 



