Van Tuyl — New Points on the Origin of Dolomite. 255 



two distinct layers each 3 feet thick. Samples of the dolo- 

 mite at the point where it passes into limestone yielded 18*1 

 per cent of MgCO, while the limestone itself yielded only - 83. 



It will he noted that in the above instances the gradation 

 of limestone into dolomite is abrupt, but in many cases the 

 gradation takes place through transition zones of limestone 

 mottled with dolomite. There can be no doubt but that these 

 mottled limestones represent an incipient stage in the process 

 of dolomitization and it is believed that many dolomites have 

 passed through such a stage in the progress of their formation. 

 In most cases the phenomenon of mottling appears to be of 

 purely inorganic origin, having resulted from a process of dolo- 

 mitization which began at certain favorable centers and spread 

 outwards. In some cases, however, it has been produced by 

 the selectine alteration of areas suggesting algse and fucoids in 

 the limestone first, and the spreading out of the dolomite from 

 these as nuclei. The Tribes Hill limestone, as developed at 

 Canajobarie, New York, furnishes an excellent illustration of 

 the mottling produced by the latter method. All stages of 

 mottling from altered fucoid-like markings to a rock uniformly 

 dolomitic may be traced in this. 



It has been observed that the spreading of dolomitization 

 from certain centers in a limestone may give rise to mottling 

 on a large scale if these centers be few and far apart. For 

 example there is a conspicuous bed of dolomite pseudo-bowlders 

 in the St. Louis limestone at Alton, 111., which appears to have 

 been formed entirely in this manner. These bowlder-like 

 masses range from a few inches up to six feet in diameter and 

 contain 32 - 39 per cent of MgC0 3 while the limestone matrix 

 bears only 3'39. That they were formed in place is clearly 

 indicated by the fact that the contact of the bowlders with the 

 limestone matrix is occasionally gradational and that the strat- 

 ification lines of the limestone may at times be traced directly 

 through the bowlders. In a layer of limestone a few feet 

 above the bowlder bed here a similar process of local dolomiti- 

 zation has given rise to the development of irregular lenses of 

 dolomite. 



If has often been noted during the course of the field studies 

 that many dolomites known to be of secondary origin show 

 little or no evidence of shrinkage and porosity determinations 

 have since shown that the transformation of a limestone to 

 dolomite, even subsequent to its deposition, need not neces- 

 sarily be accompanied by a decrease in volume as pointed out 

 by JBeaumont and consistently adhered to by later writers on 

 the subject. It seems probable, therefore, that the replace- 

 ment may proceed at times according to the law of equal 

 volumes as enunciated by Lindgren (41) and that the inter- 



