Many beds of siliceous gravel are cemented together by a siliceous cement, and are called breccia; as the plumb-pudding stones of Hartfordshire, and the walls of a subterraneous temple excavated by Mr. Curzon, at Hagley near Rugely in Staffordfshire; these may have been exposed to great heat as they were immersed in water; which water under great pressure of superincumbent materials may have been rendered red- hot, as in Papin's digester; and have thus possessed powers of solution with which we are unacquainted.
8. BASALTES AND GRANITES.
Another source of siliceous stones is from the granite, or basaltes, or porphyries, which are of different hardnesses according to the materials of their composition, or to the fire they have undergone; such are the stones of Arthur's-hill near Edinburgh, of the Giant's Causway in Ireland, and of Charnwood Forest in Leicestershire; the uppermost stratum of which last seems to have been cracked either by its elevation, or by its hastily cooling after ignition by the contact of dews or snows, and thus breaks into angular fragments, such as the streets of London are paved with; or have had their angles rounded by attrition or by partial solution; and have thus formed the common paving stones or bowlers; as well as the gravel, which is often rolled into strata amid the siliceous sand-beds, which are either formed or collected in the sea.
In what manner such a mass of crystallized matter as the Giant's Causway and similar columns of basaltes, could have been raised without other volcanic appearances, may be a matter not easy to comprehend; but there is another power in nature besides that of expansile vapour which may have raised some materials which have previously been in igneous or aqueous solution; and that is the act of congelation. When the water in the experiments above related of Major Williams had by congelation thrown out the plugs from the bomb-shells, a column of ice rose from the hole of the bomb six or eight inches high. Other bodies I suspect increase in bulk which crystallize in cooling, as iron and type-metal. I remember pouring eight or ten pounds of melted brimstone into a pot to cool and was surprized to see after a little time a part of the fluid beneath break a hole in the congealed crust above it, and gradually rise into a promontory several inches high; the basaltes has many marks of fusion and of crystallization and may thence, as well as many other kinds of rocks, as of spar, marble, petrosilex, jasper, &c. have been raised by the power of congelation, a power whose quantity has not yet been ascertained, and perhaps greater and more universal than that of vapours expanded by heat. These basaltic columns rise sometimes out of mountains of granite itself, as mentioned by Dr. Beddoes, (Phil. Transact. Vol. LXXX.) and as they seem to consist of similar materials more completely fused, there is still greater reason to believe them to have been elevated in the cooling or crystallization of the mass. See note XXIV.
NOTE XX.—CLAY.
Whence ductile Clays in wide expansion spread, Soft as the Cygnet's down, their snow-white bed.
CANTO II. l. 277.
The philosophers, who have attended to the formation of the earth, have acknowledged two great agents in producing the various changes which the terraqueous globe has undergone, and these are water and fire. Some of them have perhaps ascribed too much to one of these great agents of nature, and some to the other. They have generally agreed that the stratification of materials could only be produced from sediments or precipitations, which were previously mixed or dissolved in the sea; and that whatever effects were produced by fire were performed afterwards.
There is however great difficulty in accounting for the universal stratification of the solid globe of the earth in this manner, since many of the materials, which appear in strata, could not have been suspended in water; as the nodules of flint in chalk-beds, the extensive beds of shells, and lastly the strata of coal, clay, sand, and iron-ore, which in most coal-countries lie from five to seven times alternately stratified over each other, and none of them are soluble in water. Add to this if a solution of them or a mixture of them in water could be supposed, the cause of that solution must cease before a precipitation could commence.
1. The great masses of lava, under the various names of granite, porphyry, toadstone, moor-stone, rag, and slate, which constitute the old world, may have acquired the stratification, which some of them appear to possess, by their having been formed by successive eruptions of a fluid mass, which at different periods of antient time arose from volcanic shafts and covered each other, the surface of the interior mass of lava would cool and become solid before the superincumbent stratum was poured over it; to the same cause may be ascribed their different compositions and textures, which are scarcely the same in any two parts of the world.