S54 Prof. Keilhau on the Formation of 



as we have seen, he would also here have very willingly given 

 a volcanic explanation. His opinion regarding the metalliferous 

 veins, and the contortions of the stratified masses occurring 

 along with the concretions, has been already noticed. In an- 

 other part of his work (p. 245), he gives an ideal profile, repre- 

 senting some of these concretions, and advances the opinion 

 that, as the concretionary structure interferes with the laminae 

 of deposit, and truncates them, " there can be no doubt the 

 concretions were formed by some chemical or electric action, 

 after the first aggregation of the surrounding strata." Mur- 

 chison thinks, that the sequence of the operations was : Firsts 

 the successive deposition of the materials composing the beds ; 

 and, secondly, the arrangement of these materials, so as to occa- 

 sion particles of similar matter to unite and form concretions.* 



The second, and still more the third, of the already enu- 

 merated modes of occurrence (p. 10), of granular limestone 

 in groups of fossiliferous rocks (the first also containing or- 

 ganic remains), are likewise very instructive as regards the 

 problem of the origin of that rock. I shall here, however, con- 

 tent myself with merely mentioning them, and at once proceed 

 to the consideration of the fourth case. 



It has been observed in many localities, that beds of com- 

 pletely uncrystalline, often argillaceous, and bituminous lime- 

 stone, exhibit, when in the vicinity, or, most frequently, when 

 in direct contact with certain other rocks, an entirely different 

 constitution, inasmuch as they there consist of pure perfectly 

 saccharine granular marble. This phenomenon shews distinct- 

 ly, that, in this instance also, the marble is a later product de- 

 veloped from the already solid rough mass, and this has been 

 universally understood to be the case. 



But what is to be said farther in regard to this subject ? 



It is clear, that the contact with the dissimilar rock must 

 stand in causal connection with the alteration met with in the 

 uncrystalline limestone ; and we certainly do not err, when we 



* In this part of his work, Mr Murchison is treating more particularly of an- 

 other remarkable process, which has been in operation in the solid masses, viz. of 

 that by which, what are termed " joints," have been produced. These are the 

 parallel fissures, or ablbsungen," which, in slates, so frequently cut through the 

 true slaty structure. 



