506 STEVENSON— THE FORMATION OF COAL BEDS. [Nov. i. 



shale. Some layers are crowded with tubes resembling those of 

 annelids. The overlying sandstone contains limestone fragments 

 in the lower portion. A conglomerate limestone in Belmont county, 

 and very near the top of the Conemaugh, shows shrinkage in the 

 limestone fragments. The Monongahela limestones in the same 

 county vary from hard to soft, often becoming calcareous shale. 

 Shrinkage cracks are so numerous at times as to give a brecciated 

 structure. These limestones contain several species of minute 

 ostracoids which are very abundant, and in some beds one species 

 of ostracoid is well represented. These cover the bedding planes 

 and are shown especially well on sun-cracked surfaces Con- 

 glomerate layers are frequent, varying in 'thickness from half an 

 inch to 3 or 4 inches, and the fragments are from pea-size to several 

 inches in diameter. Hyde thinks the deposits are of freshwater 

 origin like the freshwater limestones in some of the western states 

 and he conceives that they represent the calcareous mud laid down 

 in probably shallow bodies of water. During the summer, the water 

 was evaporated and the ostracoids with the Spirorhcs were left on 

 the muddy surface, which, exposed to the heat, became sun-cracked. 

 A similar explanation was suggested by Haast''^ who, in describing 

 some Tertiary marls, says that they must have been " left high and 

 dry, exposed to the efifects of a powerful sun, is well shown by 

 numerous cracks in the clay marls, which are several inches wide 

 and deep." 



Brecciated limestones have been reported frequently from other 

 lands and the explanations ofifered are not wholly concordant. 

 Roeder''*' found in the Lancashire field a succession of red shales 

 and clays with thin coals and two limestones, in all somewhat less 

 than 900 feet thick. At Slade lane he saw 206 feet of measures, 

 mostly red, green or variegated shales with in all 21 feet 4 inches of 

 limestone. Some of the limestones are brecciated locally and the 

 passage from breccia to the normal structure is gradual. The frag- 



"'J. Haast, "Report on Geology of the Malvern Hills, Canterbury," 

 Rep. Geol. Surv. N. Z., 1872, pp. 63, 64. 



■ '"' C. Roeder, " Notes on the Upper Coal Measures at Slade Lane, Burn- 

 age," Trans. Manch. Geol. Soc, Vol. XXL, 1890, separate, pp. y-22. 



