PALEOZOIC GROUP. 295 



is possible, although we cannot at present make good paleontologic separa- 

 tions to correspond with the stratigraphy. Much of what follows upon this 

 subject must necessarily be held provisionally. 



(1.) THE LEON SERIES (CANADIAN?). 



The Llano and San Saba rivers run for many miles in parts of Mason and 

 McCulloch counties through canyons cut in dolomites, the upper layers being 

 pure white to gray and light buff, compact and very tough, underlaid by 

 shaly dark buff dolomites, and these again by thick siliceous magnesian lime- 

 stones, weathering black and roughly pitted upon surface exposures. These 

 last beds lie conformably upon another set of buff or yellow dolomites, per- 

 haps three hundred feet in maximum thickness, which are more sandy in 

 character and of shaly structure. The relations of these strata are such as to 

 make it probable that they are basal Silurian members in this region, al- 

 though our investigations have not extended over all the district in which 

 they occur. 



There is a peculiarity, as yet unexplained, which makes the co-ordination 

 of the north and south border sections a difficult matter. This is the irregu 

 larity of the Cambrian surface contact. Not only do the later sediments en 

 croach upon the eroded foundation in such manner as to present contacts 

 with almost every Cambrian horizon, successively, in different places; but 

 the various Silurian strata have also individual basal contacts with the Cam- 

 brian. In addition to this, the direct tracing of the section is rendered 

 nearly impossible by frequent breaks in continuity due to downthrow faults. 



The local name Leon Series is proposed tentatively to include all the sets 

 mentioned, which thus taken together, may be generally described as silice- 

 ous magnesian limestones of the common physiognomy of the Canadian rocks 

 of other regions. This whole series is well exposed along the lower valley of 

 Leon Creek, in Mason County. 



As a skeleton for future work the following division names are announced : 



A. The Beaver Division, or Lower; B. The Wyo Division, or Middle; and 

 C. The Hoover Division, or Upper. 



A. THE BEAVER DIVISION (LOWER CANADIAN?). 



The basal buff dolomites are a prominent feature of the scenery in the river 

 canyons wherever they are exposed, and of the great escarpment upon the left 

 bank of the Colorado, north of Morgan Creek, and near the mouth of Beaver 

 Creek. They appear also in partial exposures upon Cold Creek and in the 

 east-west escarpment following near the north line of Llano County. Out- 

 crops in similar cliffs, protected by the massive blue limestones, occur in fault 

 lines and erosion scarps in the canyons of the Llano River and of Little Bluff, 



