396 Dr. M'Culloch on a Vein in Limestone. 



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 analo- 

 gous to that state which these earths, and notoriously silica, are sub- 

 ject 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 

 the fissure is wider, crystals are seen approaching into contact, while 

 further on, the walls of the crack are lined with the first efflores- 

 cence of carbonat of lime, an efflorescence destined at no long pe- 

 riod to cement and reunite the whole. Water charged with carbo- 

 nat 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 pro- 

 duced in a similar manner, and that many at least of the onyx peb- 

 bles owe their origin to a similar cause. There is equally little dif- 

 ficulty in explaining by the same process the supposed obscure 

 septaria ; where the contraction of the softer parts of the compound 

 maes has left cavities defining those obscurely columnar forms which 



