190 Dr. BucKLAND on the Secondary Formations between Nice, ^c. 



of the red conglomerate at Exeter, and of the rothe-todte-liegende at the base 

 of the Hartz. 



Mr. De la Beche has pointed out, in the neighbourhood of Nice, details 

 which confirm the statement made in my paper on the Alps above alluded to ; 

 namely, that limestones of all formations occasionally become dolomitic, and that 

 the occasional presence of dolomite will of itself enable us to infer nothing as 

 to the age of the strata in which it may occur. Still I think it will be found 

 that in formations more recent than the new red sandstone, it assumes only an 

 accidental place and subordinate character amongst beds whose greater mass 

 still retains the condition of common carbonate of lime; whilst, in the new red 

 sandstone formation, the calcareous beds are almost exclusively dolomitic, and 

 the dolomite is of a most strongly marked and decided character. Moreover, 

 I cannot entirely acquiesce with him in the theory of M. Von Buch, which 

 attributes the origin of the magnesia in the great dolomitic mountains of the 

 Tyrol and other localities to the agency of subjacent pyroxenic rocks. I have 

 long been aware that at their point of contact with beds and dykes of trap, 

 limestones of all ages are occasionally impregnated with magnesia, and 

 become dolomitic to a small distance only from the pyroxenic rocks : but I 

 see no reason from hence to argue that strata of many hundred feet in thick- 

 ness, and at the distance of many miles from any known pyroxenic rocks, 

 have derived their magnesia from subjacent igneous rocks, where we have no 

 proof of their existence. Nor do I see how, on this hypothesis, we could ever 

 find strata of carbonate of lime alternating with beds of dolomite; for, how 

 in cases of such alternations could the superior beds have derived their mag- 

 nesia from beneath, without the inferior calcareous beds also being equally, 

 or in a greater degree, converted into dolomite? and yet examples of these 

 alternations occur abundantly in all formations, and present an objection 

 which it behoves the supporters of the theory of Von Buch upon this subject 

 to explain. 



