274 Prof. Robert C. Wallace — Gypsum and Anhydrite. 



and in similar cases is whether the anhydrite was originally deposited, 

 or was formed by transformation at a later stage, when, owing to 

 increasing depth of sedimentation, a temperature of 30° C. was 

 undoubtedly reached. 



In seeking to explain the origin of deposits such as these, one may 

 consider the two alternatives that are generally offered : First, the 

 theory of Ochsenius,^ that the salts have been deposited from an arm 

 of the sea, which is in only partial communication with the outer ocean ; 

 second, the theory that limestones have been converted into the 

 sulphates by the action of water containing S 0^ ions. In the case 

 under consideration the gypsum does not grade laterally into lime- 

 stone : basal red beds occur, and one complex sulphate at least has 

 been found. Such facts are not in accord with the theory that the 

 beds were originally limestones. On the other hand, the sequence 

 gypsum, anhydrite, and gypsum (with glauberite) is explicable on 

 the assumption of an evaporating basin, separated from the outer 

 ocean by a bar over which calcium salts might reach the inner 

 evaporating pan. The more soluble chlorides of sodium and potassium, 

 sulphates of magnesium and complex salts, have not been found in 

 this locality ; but brines which reach the surface in Upper Devonian 

 limestones contain a high percentage of potash, and may be 

 genetically connected with this horizon. 



The applicability of Ochsenius' hypothesis to the Stassf art salts has 

 recently been questioned by Walther,^ who believes that the salts 

 were precipitated from an inland basin entirely cut off from the sea, 

 that, as the shallower parts of the lake dried up, the salts were 

 redissolved and carried down to the deep basins, and that only sucli 

 deep basins are represented by the present distribution of the deposits. 

 Grabau' has made a still wider application of the desert hypothesis in 

 his explanation of the Salina deposits of JN^ew York and Michigan. 

 He considers that the land surface of the Niagara Limestones, 

 reworked by weathering agents under desert conditions, supplied — 

 mainly through the connate v/aters with which the limestones were 

 impregnated — the salts which were finally carried down, by streams 

 due to intermittent rains, into circumscribed desert basins. The 

 annual rings at Stassfurt might be accounted for, according to 

 Walther, as due to salts carried down after successive rainfalls ; and 

 the same authority considers that the absence of fossils cannot be 

 explained if communication with the open sea remained established 

 during the whole period of precipitation. In the Gypsumville deposits 

 there ai'e no annual rings; no indication of subaerial weathei'ing has 

 been found in the underlying rock ; the homogeneous character of the 

 heavy anhydrite beds is difficult of explanation on a desert erosion 

 hypothesis ; and the absence of fossils in the anhydrite-gypsum section 

 might be accounted for, as at Stassfurt,* by postulating a rather 

 extended bar where communication was made with the outer ocean, 



^ Die BUdung der Steinsalzlager und Hirer Mutterlangensalze , 1877. 

 ^ Lehrbuch der Geologie von Deutschland, 1910. 



^ Mining and Metallurgical Society of America, Bulletin 57, p. 39 ; also 

 Principles of Stratigraphy, 1913, oh. ix. 

 * Arrhenius & Lachmann, I.e., p. 142. 



