296 CENTRAL MINERAL REGION OF TEXAS. 



Bluff, Mill, Rocky, and Leon creeks, in the southern part of Mason County, 

 as well as in tributaries on both sides of the San Saba River, in San Saba, 

 McCulloch, and Mason counties. 



The Beaver Division, as here denned, is intended to include the lowest two 

 lithologic members of the Leon series, viz.: (1), the Cavern Subdivision, and 

 (2), the Bluff Subdivision. 



(1) The Cavern Subdivision, as its name implies, is often pierced, in the 

 cliffs, by small caves and grottoes, and chambers of larger size are frequent in 

 the inaccessible precipices. Some caverns of large dimensions are known in 

 districts where these rocks cover large areas. Several of these have been ex- 

 plored in Burnet County, and one which may be in this stratigraphic division 

 has become noted commercially as a large producer of "bat guano." This 

 will be described hereafter. 



The rocks of this set are chiefly the yellow sandy dolomites of the Silurian 

 base, but in the upper third of the exposures there is usually an interpolation 

 of a few feet of dark dolomite, similar to the next higher subdivision. In 

 some cases the transition is gradual enough to prevent establishing any but 

 an arbitrary line between these two sets; in other places the distinction is 

 more obvious, and, rarely, there may be a sudden change of character. 

 Although there are thus these minor discrepancies, due to the fluctuations of 

 shore lines during the deposition of the Beaver strata, and although the gen- 

 eral result was a transfer of parts of the district from near-shore to deep sea 

 conditions, there is such a constant association of the Cavern and Bluff Beds 

 in the sections, and such a close conformity between them, that they seem to 

 belong together in any appropriate classification. 



The yellow rocks belonging to the Cavern Subdivision are not wholly sandy, 

 but there are many of them which have a fine-grained arenaceous character, 

 while some of them have irregularly alternating layers of calcareous and 

 siliceous composition. Like all the rocks of the Leon Series, they are, first 

 of all, magnesian limestones; but they have enough of the sandy nature, 

 especially in the nearest contacts with the Potsdam sandstones, to make their 

 weathering, in form and talus, suggest an approach to sandstone characters. 

 This is less observable in the higher strata as a rule. Three hundred feet is 

 probably a liberal allowance for the thickness, including the massive blue 

 dolomite and the yellow overlying beds immediately below the Bluff Beds. 



There may be some lithologic justification for correlating the Cavern strata 

 with the lowest Calciferous rocks (Calciferous sandrock) of Missouri, as has 

 been done by Shumard and Glenn, but such generalization, to my mind, is 

 not in keeping with the spirit of modern science. The fossils obtained have 

 not been numerous, and very few of them are in a good state of preservation. 

 Fragments of Trilobites from a Cold Creek section may be Bathyurus; some 



