300 CENTRAL MINERAL RECHON OF TEXAS. 



wherever the crag dolomites have been observed beneath beds of the Hoover 

 facies, there is a set of fossil-bearing beds. With more or less of variety in 

 the different localities, and with no certainty that the fossil strata, or any of 

 them, will be found at the Hoover contacts with the Potsdam, no "Wyo-Hoover 

 contact has yet been observed without a f ossiliferous layer. A set of fucoid- 

 bearing beds above a wave-marked set affords the most suitable dividing line 

 for our purpose. 



Beginning with the dark f ucoidal limestone, the Hoover Division gradually 

 passes upward through thin, slabby, fossiliferous limestone of the compact 

 Burnet marble texture, becoming thicker, more pure, and lighter colored 

 above, into fine-grained, gritty, crystalline dolomites, forming transition beds 

 to the cherty San Saba series above. 



The fucoid beds are thin, brittle, and of irregular texture. In the James 

 River valley, in Mason County, near the Kimble County line, these rocks 

 cover a very wide area, and they are also displayed as the surface terrane 

 over much of the San Saba valley in Mason County. The markings are dis- 

 posed in graceful curves, and they seem to correspond in generic characters 

 with Bythotrephis, Hall, but the large size, habit of growth, and other specific 

 features remove it from B. gracilis, Hall, which it resembles in a very general 

 way. Above these, in the whiter layers, sometimes parts of the same or ad- 

 joining slabs, are very abundant coatings of a reddish brown, branching 

 fucoid? of different character. This more closely agrees with Bythotrephis 

 succulens, Hall, and it occurs in excellent shape for study. These fucoids lie 

 in interstices of the wavy lamination, so that they weather out to the best ad- 

 vantage. Still higher, but usually beginning near the fucoid al beds, there 

 are grey and dull brownish "Burnet Marbles" containing animal remains, 

 largely Gasteropods, but with occasional forms like Eocystites and fragments 

 of small undetermined crinoids, etc. One prominent form of G-asteropod, 

 appearing in section at the surface of the weathered grey beds, is a Maclurea, 

 near M. logani, Salter. Upon Arrott's Branch of Little Llano Creek and 

 farther south, the partly calcareous Wyo beds are separated from the Hoover 

 Division by a kind of chalky limestone, largely made up of globules with 

 almost indeterminate markings; These are a little variable in size, averaging 

 perhaps one-fourth to one-half an inch in diameter, or more. It is possible 

 that they are Malocystites murchisoni, Billings, of the Chazy age. 



Above these fossil horizons the Hoover beds become more pure, lighter 

 colored and thicker, without lamination planes. As a rule, however, the two 

 kinds of beds do not occur in a continuous vertical section, from which we 

 may infer that they represent the nearly coincident accumulation of shallow 

 water and deep sea deposits in different localities. Some observations lead 

 to the supposition that an important stratigraphic break occurs at the summit 



