404 J. E. Hyde — Desiccation Conglomerates. 



relation to the blocks produced by cracking of a mud surface. 

 The latter type is not yet explained. 



The origin of these limestones is an open question, but it is 

 probable that they were formed in restricted basins which 

 may have had an extent of many miles. The numerous mud- 

 cracked surfaces and abrupt lithological changes show that the 

 area was subject either to rapid oscillation and consequent suc- 

 cessive flooding and drainage, if they be considered of marine 

 origin, or to periods of desiccation if they be considered as lake 

 deposits. There is not a single species suggesting an undoubt- 

 edly marine fauna (unless Spirorbis be so considered) and the 

 beds were probably deposited in fresh or brackish lakes. The 

 fresh marl lakes occurring throughout much of the drift-cov- 

 ered area of North America suggest a method of formation 

 which, however, possibly differed considerably in details. In 

 these lakes deposits of calcium carbonate are forming at pres- 

 ent as a result of precipitation of lime by aquatic plants, prin- 

 cipally Char a. These beds in places are known to be at least 

 30 to 45 feet or even more in thickness and may consist of 

 quite pure lime or they may be quite highly magnesian with 

 considerable clay and vegetable matter. During the summer 

 months the water of some of the lakes evaporates so far as to 

 leave great flats of almost pure lime mud exposed to the 

 atmosphere and these in drying become cracked.* TJnfor- 



* Geol. Surv. Michigan, vol. viii, pt. iii, pp. 41-96. 25th Ann. Rept. 

 Dept. Geol. and Nat. Resources, Indiana. On plate 6, fig. a, opp. p. 41, a 

 photograph is given showing such a nrad-cracked surface. 



Since this paper was completed Mr. George H. Ashley has offered the same 

 suggestion for the origin of most of the limestones of the Appalachian coal 

 fields, including many of those lying below the Ames limestone. (' ' Were the 

 Appalachian and Eastern Interior Coal Fields Ever Connected ?" Economic 

 Geology, vol. ii, pp. 659-666.) He says in part : '"It is a singular fact that 

 in the Appalachian basin the majority of the limestones underlie the nearest 

 of the associated coals, as note the Vanport, the Johnstown cement, Upper 

 and Lower Freeport, Pittsburg and other limestones, all of which closely 

 underlie the corresponding coals," and '"it may almost be termed a habit of 

 the limestones to cany an overlying bed of iron ore" (p. 664). 



In explanation of this succession the following is proposed : ' 'The lime- 

 stones of the Appalachian field I ascribe largely to the action of plants and 

 bacteria, their accumulation being in bogs or enclosed waters by secretion of 

 lime from the water, as is being done to-day in the lakes and marshes of 

 northern Indiana, Ohio and southern Michigan. The iron ores were accumu- 

 lated under similar conditions by the same or associated bacteria in the same 

 way that bog ores are accumulating to-day in the localities named and in 

 many parts of the world. These accumulations were followed by the growth 

 of a peaty deposit that later became coal" (p. 665). 



It is not evident how far Mr. Ashley intends to apply this explanation. 

 In Ohio, contrary to his statement, the four best known limestones of the 

 lower Coal-rneasures. the Lower and Upper Mercer, Putnam Hill, and Fer- 

 riferous or Vanport limestone, directly or almost directly overlie the nearest 

 coal, and each carries a more or less persistent iron ore. The same is true 

 of the Cambridge limestone horizon in Muskingum county, Ohio, except that 

 the iron ore is not present. The association of coal, limestone and iron ore 

 is just as intimate as in the examples which he cites although the order of 

 superposition is different, but the limestones are unquestionably of marine 

 origin and will not fall under his explanation. 



