VanTuyl — New Points on the Origin of Dolomite. 249 



Art. XXVIII. — New Points on the Origin of Dolomite :* by 

 Francis M. VanTuyl. 



Historical Review. 



The problem of the origin of dolomite has long occupied the 

 attention of geologists and many theories have been advanced 

 for its formation, but no one of these theories has been widely 

 accepted. Von Bitch (l)f was the first to seriously attempt to 

 explain the formation of the rock. As early as 1822 in his 

 writings on the dolomite of the Tyrol, he ascribed its origin to 

 the action of volcanic vapors, rich in magnesia, on limestone, 

 and there was some basis for this belief, for the rocks are there 

 penetrated by augite-porphyry. Frapolli(2) and Durocher (3) 

 later expressed similar views upon the origin of the rock, and 

 Favre (4), basing his supposition upon the conditions of the 

 experimental production of dolomite by Marignac, concluded 

 that the dolomite of the Tyrol was formed by the alteration of 

 limestone beneath the sea at a temperature of 200° C. and at a 

 pressure of 15 atmospheres, corresponding to a depth of 150 to 

 200 meters, by magnesium compounds furnished by the action 

 of sulphurous and hydrochloric acids of volcanic origin on the 

 lava of submarine melaphyr eruptions. 



In 1834 Collegno (5) pointed out the frequent association of 

 gypsum and dolomite in the St. Gothard region and regarded 

 them both as transformation products resulting from the action 

 of magnesium sulphate in surface waters or limestone. Mor- 

 lot(6) also favored such a theory of origin. 



As early as 1836 Beaumont (7) ascribed the origin of dolo- 

 mite to the alteration of limestone by circulating solutions of 

 magnesium bicarbonate and, assuming that the replacement was 

 molecular, he calculated that the change should be accompanied 

 by a decrease in volume of the original rock to the extent of 

 about 12'1 per cent. Actual porosity determinations by Mor- 

 lot (8) on a dolomite sample from the Alps later seemed to con- 

 firm this prediction. 



In 1843, A. W. Jackson (9) suggested that ascending spring- 

 water bearing magnesium bicarbonate might effect the change. 

 Nauck, Haussmann, Bischof, Zirkel and others, however, 

 subscribed to the view that ordinary circulating ground water 

 bearing magnesium bicarbonate had attacked the limestone. 



* The present article is based on a more extended paper which constitutes 

 a portion of volume xxv of the Iowa Geological Survey. The reader is 

 referred to this report for details. 



f For numbered references to the literature, see the list at the end of this 

 article. 



