304 CENTRAL MINERAL REGION OF TEXAS. 



3. Thinly laminated, tough, cherty dolomites, white to dull grey, or 

 chalky, sometimes with interstratified beds of chert, from one foot to three 

 feet in thickness. Maximum thickness not above 25 feet. 



4. Massive chert beds, averaging about ten feet or less, overlaid by 



5. Fossiliferous strata of varying thickness and difficult of interpretation. 

 The best judgment the writer can offer is that a set of essentially cherty beds, 

 now largely decomposed, and represented by fragmental surface deposits, 

 apparently in situ for the most part, originally made up a considerable part of 

 the Silurian system. At present the total thickness of this cap is probably 

 50 to 100 feet, although it would be difficult to accept this statement without 

 a knowledge born of actual day by day experience over many square miles of 

 territory. The strata represented by this vast aggregation of debris appear 

 to have been of the following kinds, beginning below: 



5a. Dolomitic, light-colored siliceous rocks, containing abundant remains 

 of Stromatopora sp. f , or of one form or more of Bryozoa. 



5b. Much thicker cherty strata, carrying perhaps some dolomite, but now 

 represented by masses of cauliflower or sponge-like ramifications of chert and 

 drusy quartz. For want of better terms I have given these the distinctive 

 working titles of " spongy chert " and "spongy quartz." The exact relations 

 of the two and each to a third form, a compact fossiliferous chert, are as yet 

 unknown. In fact the whole problem relating to the debris awaits a solution 

 which I hope to be able to work out the coming season. To the casual ob- 

 server the surface capping of the highest Silurian would, no doubt, suggest 

 the idea of a drift deposit. And there are facts which go to support the 

 hypothesis of transportation to a moderate distance. The chief argument of 

 this nature rests upon the truth that, upon the southern border of the debris 

 area, the fragements commonly rest upon strata much lower than those of an 

 age directly antecedent to themselves. There may, perhaps, also be a thin- 

 ning of the "float" towards the south, and possibly a predominance of the 

 material of the higher strata in that direction. But without special investi- 

 gation upon these points, a correct estimate of their importance can not be 

 made. Granting these premises, however, it is not to be gainsaid that the 

 whole- San Saba section is less complete, thinner, and more deficient in the 

 higher beds than is the case farther north. It is also true that no instance 

 has yet been observed of the mingling of the different kinds of debris in one 

 surface exposure. Wherever such occurrences have been noted there is 

 always evidence of strictly local transportation. More than this, rare out- 

 crops of the actual undecomposed strata in question have been encountered, 

 and they always sustain the same relations, one to another, as those existing 

 among the debris layers. The shattered condition of these fragmental de- 

 posits, as compared with the massive structure of the supposed original 



