508 STEVENSON— THE FORMATION OF COAL BEDS. [Nov. i, 



posure after the limestone had hecome consolidated; the features in 

 most cases are those observed where a moist material, still soft, 

 is exposed to the air ; the brecciation is that of cracking due to loss 

 of moisture. Haast's explanation, more or less modified by later 

 students, is evidently the one most closely related to the facts. 



Perhaps some reader may hesitate to believe that limestones as 

 thick as those of western Pennsylvania can be of freshwater origin; 

 but they are not greater than some recent marl deposits within the 

 United States. Davis''^ has discussed the whole subject showing the 

 mode in which the material accumulates and its relation to peat 

 deposits. D. J. Hale, in the same volume, described many ^Michigan 

 deposits, 20 to 47 feet thick. Blatchley and Ashley^"" have shown 

 that conditions are the same in Indiana as in Michigan. They are 

 convinced that the supply of calcareous matter was ample in later 

 Coal Measures times, for limestones must have cropped out in 

 extensive areas and the springs in much of the region must have 

 been charged with dissolved limestone. But both the Appalachian 

 limestones and the western marls are insignificant when compared 

 with those of the Tertiary lignite area of I'Aude in France."^ The 

 section at Cavonetti shows at top a freshwater limestone, nearly 80 

 meters thick. Some of its beds are rich in river shells and others 

 are equally rich in Lymncea and Planorbis, which are found also in 

 the shales and in the lignite. A similar limestone, 10 to 15 meters 

 thick, is lower in the section and the lowest member exposed is of 

 the same type. 



The presence of Spirorbis has been regarded by some as an 

 objection to acceptance of freshwater origin for the limestones, for 

 beyond all doubt that form is related to marine types. It occurs 

 throughout the Coal Measures column, sometimes in deposits which 

 are certainly marine and at other times in deposits which are dis- 



""C. A. Davis, "A Contribution to the Natural History of Marl," Geol. 

 Surv. Mich., Vol. , VIII., Pt. III., 1903, PP. 65-96. 



'""W. S. Blatchley and G. H. Ashley, "The Lakes of Northern Indiana 

 and their Associated Marl Deposits," Twenty-fifth Ann. Rep. Geol. Surv. 

 Ind., 1901. 



•""(M) de Serres, "Observations geologiques sur le Departement de 

 I'Aude," Soc. dcs Sci. Lille, 1835. PP- 453-455- 



I 



