CONSOLIDATION OF STKATA. 
63 
frozen ; because one ingredient of the mass, namely, the wa¬ 
ter, has crystallized, so as to hold firmly together all the sep¬ 
arate particles of which the loose mud and sand were com¬ 
posed. 
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 cabin¬ 
ets, 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 hajjpen in the case of the beryl.* 
The marl recently deposited at the bottom of Lake Supe¬ 
rior, in North America, is soft, and often filled with fresh¬ 
water shells; but if a piece be taken up and dried, it be¬ 
comes 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, containing fresh-water shells. 
Concretionary Structure. —It is probable that some of the 
heterogeneous materials wdiich rivers transport to the sea 
may at once set under water, like the artificial mixture call¬ 
ed pozzolana, which consists of fine volcanic sand charged 
with about twenty per cent, of oxide of iron, and the addi¬ 
tion 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 buildings in the 
sea. Consolidation in such cases is brought about by the ac¬ 
tion of chemical affinity on finely comminuted matter previ¬ 
ously suspended in water. After deposition similar particles 
seem often to exert a mutual attraction on each other, and 
congregate together in particular spots, forming lumps, nod¬ 
ules, 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 laminae are often trace¬ 
able through the concretions, 
remaining parallel to those of the surrounding unconsoli¬ 
dated rock. (See Fig. 48.) Such nodules of limestone have 
often a shell or other foreign body in the centre. 
Among the most remarkable examples of concretionary 
structure are those described by Professor Sedgwick as 
* Dr. MacCulloch, Syst. of Geol., vol. i., p. 123. 
Fig. 48. 
Calcareous nodules in Lias. 
