800 A TREATISE ON METAMORPHISM. 



are often dolomite. Where the rock is dolomite the fossils and the stratifi- 

 cation are obscured or destroyed. The deeper lying rock, as well as the 

 rock remote from the joints, is ordinary limestone." Geikie says that 

 essentially the same facts are true on a large scale in the Carboniferous 

 limestone in the north of England. 6 



Spurr describes the limestone of Aspen Mountain, Colorado, as dolomite 

 near the faults. The magnesium becomes less and less plentiful as "the 

 distance from the fractures increases. He further states that dolomite 

 almost invariably accompanies the ore." Bain describes precisely similar 

 phenomena in the limestone carrying lead and zinc ore in Iowa/ Spurr 

 further finds in the limestone of Grlenwood Springs, Colo., old water 

 channels and zones of fracture; and adjacent to these he finds the rock 

 to approach dolomite. Adjacent to the fractures and the underground 

 water channels there is a gradation of the dolomite to the ordinary lime- 

 stone/ Bain states that, the Carboniferous limestones of Missouri bearing 

 lead and zinc ore are dolomite in the vicinity of the ore deposits, and 

 elsewhere are largely limestone. He further finds that dolomite is a most 

 important gangue mineral in the openings of the rocks. The fissures con- 

 nect with the magnesium-bearing Cambro-Silurian limestones below, and he 

 infers that rising waters have transported the magnesium and dolomitized 

 the Carboniferous limestone adjacent to the water channels/ Other 

 instances of rapid gradation in the amount of dolomite are furnished by 

 limestones where cut by intrusives. Fiually, there are gradual variations 

 in the amount of magnesium in great limestone formations ; as, for instance, 

 the great Cambro-Silurian limestone of North America. The Appalachian 

 Valley Cambro-Silurian limestone is a true dolomite, while at various places 

 in the Mississippi Valley the limestone contains so little magnesium as to be 

 merely a magnesian limestone. 



The different cases of occurrence of magnesian limestone above given 



« Prestwich, Joseph, Geology — chemical, physical, and stratigraphical, Oxford, 1886, vol., 1, pp. 

 113-114. 



6 Geikie, Archibald, Textbook of Geology, 3d ed., Macmillan & Co., London, 1893, p. 321. 



o Spurr, J. E., Geology of the Aspen mining district, Colorado: Mon. II. S. Geol. Survey, vol. 31, 

 1898, pp. 210-211. 



<J Calvin, Samuel, and Bain, H. F., Geology of Dubuque County: Iowa Geol. Survey, vol. 10, 1900, 

 pp. 492-198, 572-575. 



<• Spurr, cit., pp. 212-216. 



/Bain, H. F., with Van Hise, C. R., and Adams, G. I., Preliminary report on the lead and zinc 

 deposits of the Ozark region: Twenty-second Ann. Rept, U. S. Geol. Survey, pt. 2, 1901, pp. 208-210. 



