Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/406



The solution of these earths in water is unquestionably more perfect in nature and the solutions more saturated than those which we can produce in our laboratories. Doubtless there is a state of division which renders them thus easily soluble, and which is perfectly analogous to that state which these earths, and notoriously silica, are subject to even in our little experiments. The gradual formation of quartz veins is too slow perhaps to be witnessed, but it may be conjectured from the various states in which they are seen, sometimes forming a detached and distinct crystallization, at others a solid mass, and visible more particularly in the schistose rocks. In limestone, the progress being more rapid, is more obvious. In the marble beds of Glen Tilt admirable examples of this process are to be seen. Fissures are here of common occurrence in the exposed layers. If we examine these, the thinner parts are found filled with a solid mass of crystallized carbonat of lime. Towards the center, where a the fissure is wider, crystals are seen approaching into contact, while further on, the walls of the crack are lined with the first efflorescence of carbonat of lime, an efflorescence destined at no long period to cement and reunite the whole. Water charged with carbonat of lime is also found in the cavities when a successful fracture of them can be obtained.

This then is the secretion by which these veins are filled up, and it offers a demonstration of which the several steps are as perfect as if we actually saw them succeeding each other. There is no reason to doubt that the stalactitical chalcedonies of the trap rocks are produced in a similar manner, and that many at least of the onyx pebbles owe their origin to a similar cause. There is equally little difficulty in explaining by the same process the supposed obscure septaria; where the contraction of the softer parts of the compound mass has left cavities defining those obscurely columnar forms which