DOLOMITIZATION BY GROUND WATERS. 807 



foregoing-, one would expect to find dolomites among- those rocks which 

 have been deeply buried and strongly deformed, but at the time of dolo- 

 mitization not so deeply buried as to prevent deformation from producing 

 fractures for circulation, for there, if anywhere, the process of dolomitiza- 

 tion would have been nearly completed. With this expectation the facts of 

 occurrence of dolomite as given on pages 798-802 fully correspond. 



As to whether dolomitization takes place more rapidly while the solu- 

 tions are descending or ascending no definite answer can be gn r en without 

 experimental woi'k. It is believed that the process probably occurs 

 throughout the journey of the underground water, as explained on pages 

 636-639. Doubtless in many cases where there is local dolomitization 

 adjacent to the main underground circulation channels, the process has 

 taken place while the waters were ascending, for, as shown on page 583, 

 the trunk channels of underground waters are more often ascending than 

 descending. Excellent illustrations of local dolomitization by ascending- 

 waters are those already referred to at Glenwood Springs, Colo., 

 described by Spurr," and in the Carboniferous limestones of Missouri, 

 adjacent to the lead and zinc areas described by Bain.'' But in many cases 

 of dolomitization along- joints and other fractures the water was doubtless 

 descending. The jn-ocess ma} - have been accomplished by waters which 

 were ascending at one time and descending at another time. For instance, 

 when the rock was somewhat deeply buried, a channel may have been 

 occupied by ascending waters, and when brought nearer to the surface by 

 erosion this same channel may have been occupied by descending waters, 

 and dolomitization continued throughout the process. 



Various regions could be mentioned which illustrate the above- 

 mentioned conditions for dolomitization. One of the best of these is that 

 portion of the limestones of the Mississippi Valley which is in the belt of 

 cementation. It is well known that these limestones have been broken at 

 various times so as to produce joint fractures and local faults, and thus 

 furnish channels for readv circulation. In the belt of weathering- the gre^t 

 Cambro-Silurian limestone, which contains magnesium, and the magnesium 

 of the other rocks is continuously being- dissolved and carried downward 



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

 1898, pp. 212-216. 



& Bain, H. F., Van Hise, C. R., and Adams, G. 1., 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. 



