370 CENTRAL MINERAL REGION OF TEXAS. 



of character of this kind of product, but we may get important hints from 

 such observations as have heretofore been possible. 



A. THE FEBNANDAN BLUE MABBLES. 



I do not think it probable that the oldest blue marbles (Fernandan) 

 occurring in the northwest trend, in connection with the magnetic iron 

 belts, will be found satisfactory for anything but a limited range of applica- 

 tion. They are usually thin and much broken by joints, their dark streaky 

 color is objectionable for most uses, and they are often pyritous. This last 

 feature renders them liable to stain badly upon exposure, owing to decom- 

 position of the pyrite. Careful selection, after much prospecting at some 

 depth beneath the surface, may prove that these opinions are unfounded, but 

 it will be wise for those who desire to use the material to get expert advice 

 as to the probable effects of long exposure. 



B. THE TEXAN WHITE MABBLES. 



Much more promising are the white marbles of the Texan System, cours- 

 ing north and south in their earliest trend. These are often snowy white, 

 even grained, fully compact, and there appear to be strata among them of 

 medium thickness. In prospecting for these care should be taken to select 

 localities in which the mendianal trend is prominent, as in the district south 

 of Packsaddle Mountain, and perhaps more surely in the Comanche Creek 

 Region, in Mason County. The exposures are more abundant in the for- 

 mer area, but probably less affected by the later uplifts in the latter tract. 

 It does not now seem likely that any of these marbles can be made to com- 

 pete with the more massive and more easily worked products of the Eastern 

 States, but some local application of them may be possibly made profitable 

 eventually. 



C. THE SILUBIAN MABBLES. 



There is more hope for some of the compact dolomytes of the Silurian 

 System, especially those in the Hoover Division of the Leon Series, and per- 

 haps selected beds in the San Saba Series. 



Burnet, Blanco, Gillespie, Mason, San Saba, and McCulloch counties have 

 many good exposures of the Hoover Beds. There is, however, much varia- 

 tion in quality in the different outcrops, and it is as yet uncertain where the 

 best commercial products may be raised. In testing outcrops it is very im- 

 portant to ascertain how seriously the rocks have been affected by the various 

 upheavals, because breaks which do not show in faults or master joints may 

 be dormant in the mass, ready to be developed upon exposure to the ele- 

 ments. This source of annoyance is more fully explained beyond, under the 

 head of Lithographic Stone. 



