73. Another remarkable fact in the natural history of the whinstone rocks, remains yet to be mentioned, and with it I shall conclude the argument, as far as these rocks are concerned.
Some of the species of whinstone are the common matrices of agates and chalcedonies, which lie inclosed in them in the form of round nodules. The original fluidity of these nodules is evinced by their figured, and sometimes crystallized structure, and indeed is so generally admitted, that the only question concerning them is, whether this fluidity was the effect of heat or of solution. To answer this question, Dr Hutton observes, that the formation of the concentric coats, of which the agate is usually composed, has evidently proceeded from the circumference toward the centre, the exterior coats always impressing the interior, but never the reverse. The same thing also follows from this other fact, that when there is any vacuity within the agate, it is usually at the centre, and there too are found the regular crystals, when any such have been formed. It therefore appears certain, that the progress of consolidation has been from the circumference inwards, and that the outward coats of the agate were the first to acquire solidity and hardness.
74. Now, it must be considered that these coats are highly consolidated; that they are of very pure siliceous matter, and are utterly impervious to every substance which we know of, except light and heat. It is plain, therefore, that whatever at any time, during the progress of consolidation, was contained within the coats already formed, must have remained there as long as the agate was entire, without the least possibility of escape. But nothing is found within the coats of the agate save its own substance; therefore, no extraneous substance, that is to say no solvent, was ever included within them. The fluidity of the agate was therefore simple, and unassisted by any menstruum.
In this argument, nothing appears to me wanting, that is necessary to the perfection of a physical, I had almost said of a mathematical, demonstration. It seems, indeed, to be impossible that the igneous origin of fossils could be recorded in plainer language, than by the phenomenon which has just been described.
75. The examination of particular specimens of agates and chalcedonies, affords many more arguments of the same kind, which Dr Hutton used to deduce with an acuteness and vivacity, which his friends have often listened to with great admiration and delight.[22] These, however, must be passed over at present; and I have only further to remark, that a series of the most interesting experiments, instituted by Sir James Hall, and published in the Transactions of the Royal Society of Edinburgh,[23] has removed the only remaining objection that could be urged against the igneous origin of whinstone. This objection is founded on the common observation, that when a piece of whinstone or basaltes is actually melted in a crucible, on cooling, it becomes glass, and loses its original character entirely; and from thence it was concluded, that this character had not been originally produced by fusion. The experiments above mentioned, however, have shown, in the most satisfactory manner, that melted whin, by regulated or by slow cooling, is prevented from assuming the appearance of glass, and becomes a stony substance, hardly to be distinguished from whinstone or lava.
[23] vol. v. p. 43.
The experiments of another ingenious chemist, Dr Kennedy, have shown, that whinstone contains mineral alkali, by which, of course, its fusion must have been assisted.[24] Dr Hutton used to ascribe its fusibility, in a great measure at least, to the quantity of iron contained in it: both these causes have no doubt united to render it more easily melted than the ordinary materials of the strata.
[24] Trans. R. S. Edin. vol. v. p. 85.
76. In a word, therefore, to conceive aright the origin of that class of unstratified rocks, distinguished by the name of whinstone, we must suppose, that long after the consolidation of the strata, and during the time of their elevation, the materials of the former were melted by the force of subterraneous heat, and injected among the rents and fissures of the rocks already formed. In this manner were produced the veins or dikes of whinstone; and, where circumstances allowed the stream of melted matter to diffuse itself more widely, tabular masses were formed, which were afterwards raised up, together with the surrounding strata, above the level of the sea, and have been since laid open by the operation of those causes that continually change and waste the surface of the land.