59. The want of any appearance of stratification in mineral veins has already been taken notice of. There is, however, to be observed, in many instances, a tendency to a regular arrangement of the substances contained in them; those of the same kind forming coats parallel to the sides of the vein, and nearly of an equal thickness. This phenomenon is considered as one of the strongest arguments in favour of the Neptunian system, but has nothing in it, in the least incompatible with that theory which ascribes the formation of veins to the action of subterraneous heat. When melted matter from the mineral regions was thrown up into the veins, that which was nearest to the sides would soonest lose its heat. The similar substances, also, would unite while this process was going forward, and would crystallize, as in other cases of congelation, from the sides toward the interior. There is the more reason for supposing this to have been the case, that the same sort of coating is often observed on the inside of close cavities, which are, nevertheless, so constructed, as to afford a demonstration that no chemical solvent was ever included in them, (§ 74.) Some veins, it must also be considered, may have been filled by successive injections of melted matter, and this would naturally give rise to a variety of separate incrustations.[20] [Footnote 20: See some farther remarks on this subject at NOTE XIII.] 60. In the view now given of metallic veins, they have been considered as traversing only the stratified parts of the globe. They do, however, occasionally intersect the unstratified parts, particularly the granite, the same vein often continuing its course across rocks of both kinds, without suffering any material change; and, if we have hitherto paid no attention to this circumstance, it is because the order pursued in this essay required, that the relation of the veins to stratified bodies should be first treated of. Besides, the facts in the natural history of veins, whether contained in stratified or unstratified rocks, are so nearly alike, that in a general view of geology, they do not require to be distinguished. It is material to remark, that, though metallic veins are found indiscriminately in all the different kinds of rock, whether stratified or otherwise, they are most abundant in the class of primary schisti. All the countries most remarkable for their mines, and the mountains distinguished by the name of metalliferous, are primary, and the instance of Derbyshire is perhaps the most considerable exception to this rule that is known. This preference, which the metals appear to give to the primary strata, is very consistent with Dr Hutton's theory, which represents the rocks of that order as being most changed from their original position, and those on which the disturbing forces of the subterraneous regions have acted most frequently, and with greatest energy. The primary strata are