382 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



found in the lower Cambrian in the vicinity of Silver Peak, Nevada, 

 where nearly 6,000 feet of beds referred to this division are exposed. 

 The middle Cambrian in the House Eange and Wasatch Mountains is 

 nearly 5,000 feet thick and composed almost entirely of limestone. Of 

 the limestones referred by Mr. Walcott to the upper Cambrian, aggre- 

 gating in different sections from 1,227 to 3,590 feet, a large part is 

 thought to represent the proposed Ozarkian system. According to Mr. 

 Walcott there is no physical break between the top of the "upper Cam- 

 brian" and the limestone referred by him to the Ordovician. The break 

 at the top of the "middle Cambrian," however, seems generally well 

 marked. 



The aggregate thickness of 6,500 feet ascribed to the Ozarkian system 

 is a conservative estimate made up of 4,000 feet of Knox dolomite in the 

 western part of the valley of east Tennessee plus about 2,500 feet of over- 

 lying and underlying dolomites seen in Alabama. As shown on pages 

 633 to 639 the aggregate thickness of calcareous Ozarkian deposits in the 

 southern Appalachian Valley alone seems to be nearly 8,000 feet. More 

 detailed sections will be given in one of a proposed series of papers on 

 local stratigraphic subjects. 



The thickest continuous section of calcareous deposits belonging to 

 the Canadian system is at Bellefonte, Pennsylvania, where the total 

 thickness is not less than 4,200 feet. There is reason to believe that still 

 older deposits of this period are included in the Levis shale of the Saint 

 Lawrence Valley and possibly also younger beds in the Arbuckle lime- 

 stone of Oklahoma. 



The total of 5,700 feet of Ordovician limestone is made up chiefly 

 from deposits in the Appalachian Valley. The principal exception is 

 the basal series comprising the Saint Peter sandstone. This is well devel- 

 oped in the Mississippi Valley, especially in northern Arkansas, but so 

 far is unknown in the Appalachian Valley. This basal series is given 

 a limestone value of 300 feet, though it probably represents a much 

 greater time interval than this amount implies. The oldest of the Ordo- 

 vician deposits in the Appalachian Valley is the Stones Eiver limestone, 

 of which 1,300 feet are seen at Martinsburg, West Virginia, all of it 

 presumably younger than the Saint Peter. This limestone is followed 

 by the Blount group of limestone shale and sandstone with a maximum 

 thickness of at least 3,500 feet observed in Grainger and Blount counties, 

 Tennessee. The limestone value of this group is not less than 1,500 feet. 

 Next comes the Black Eiver group of limestones comprising the equiva- 

 lents in northeastern Tennessee of the Lowville and Watertown limestones 

 of New York, of the upper Chambersburg limestone of Pennsylvania, and 



