72. If, indeed, in these instances, the gradation were insensible, as some have asserted it to be, between the strata and the interposed mass, so that it was impossible to point out the line where the one ended and the other began, whatever difficulties we might perceive in the Neptunian theory, we should find it hard to substitute a better in its room. But the truth seems to be, that, in the cases we are now treating of, no such gradation exists; and that, though where the two kinds of rock come into contact a change is often observed, by the strata having acquired an additional degree of induration, yet the line of separation is well defined, and can be precisely ascertained. This at least is certain, that innumerable specimens, exhibiting such lines of separation, are to be met with; and wherever care has been taken to obtain a fresh fracture of the stone, and to remove the effects of accidental causes, even where the two rocks are most firmly united, and most closely assimilated, I am persuaded that no uncertainty has ever remained as to the line of their separation. For these reasons, it seems probable that the gradual transition of basaltes into the adjoining strata, is in all cases imaginary, and is, in truth, a mere illusion, proceeding from hasty and inaccurate observation. 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.