258 Van Tuyl — New Points on tin Origin of Dolomite. 



the limestone a short distance in advance of the main dolomite 

 area. As the replacement proceeds the dolomite areas grow 

 larger and larger and eventually meet and become continent 

 thereby giving rise to a rock which is uniformly dolomitic. 

 Further addition of magnesia may then take place by altering 

 the rock more completely. 



In the coarse-grained limestones, especially those which were 

 originally coarse-grained, such as the crinoidal limestones, on 

 the other hand, mottling does not seein to be the rule in the 

 early stages of alteration. In these the replacement appears to 

 affect the matrix first and to spread rapidly through the rock. 

 The coarser grains are next affected, being broken down into 

 aggregates of small dolomite grains. In the end a coarse- 

 grained limestone may be changed over into a uniformly fine- 

 grained one. 



In the dolomitization of limestones of both types the calcar- 

 eous skeletons of organisms appear in most cases to successfully 

 withstand alteration and these, owing to their greater solubility 

 than dolomite, are then removed to leave molds by a process 

 of atmospheric leaching when the formation passes into the 

 belt of weathering. 



Conclusions. 



Considering all the evidence, it seems probable that the 

 great majority of our dolomites had their inception in the 

 alteration of limestones. It will not be denied, however, that 

 some dolomitic formations of minor importance may have had 

 a different origin. For instance, some impure dolomitic lime- 

 stones associated with shales very probably represent original 

 clastic deposits which have not suffered any alteration what- 

 ever, and there is some reason for believing that certain dolo- 

 mitic limestones high in siliceous material, such as the Silurian 

 waterlimes of New York State, may have had a similar origin. 



The importance of marine and surface leaching in increasing 

 the magnesia content of limestones originally low in magnesia 

 should likewise not be overlooked. There can be no doubt 

 that this process has greatly enriched the more vesicular dolo- 

 mitic limestones in magnesia. But the leaching theory does 

 not explain the ultimate source of the magnesia. It merely 

 shows how the magnesia content of a limestone originally low 

 in this constituent can be enriched. 



To return now to the dolomites which have resulted from 

 the alteration of limestone, there are many reasons for believ- 

 ing that the more extensive of these have all been formed 

 beneath the sea, and that dolomitization affected by ground 

 water is only local and very imperfect. Some of the features 



