tin. 597 



along a line bearing thence north 75° west to the valley of Herman Creek 

 and beyond, there are many places where all the conditions which exist at 

 the two extremes of the line are present and geologically identical with them. 

 We have not yet discovered cassiterite in all the intervening areas, nor can 

 it yet be determined what influence, if any, may have come from the later 

 upheavals, which have complicated the structure more there than elsewhere. 

 But there is a very apparent similitude in the occurrence of rare metals in all 

 the Burnetian outcrops wherever found. Besides this it is significant that in 

 the later cross-trends, which in wide bands have as it were cut out parts of the 

 Burnetian nucleus, there sometimes are traces of tin in the special minerals 

 which characterize the successive uplifts. The direct relations of this pecu- 

 liarity to the oldest Archean (Burnetian) axis is most pronounced, for it is only 

 where that backbone is prominent that the cross-folds are tin bearing. To 

 illustrate the point more clearly the following observations are adduced : . 



a. The uplift next succeeding the Burnetian is the Fernandian, with a 

 northwest (north 35° west) trend. This cuts out the Burnetian axis in much 

 of Llano County, where the remarkably abundant and pure magnetites and 

 hematites are well displayed. The easternmost folds of the latter system, ad- 

 joining good exposures of the earliest trend, carry magnetic iron ores of a 

 peculiar, tough, coarsely crystalline character, of high specific gravity and 

 brilliant surfaces of fracture, which yield occasional traces of tin and the rarer 

 metals. Wolframite and minerals with smaller proportions of tungsten are 

 associated. 



h. The Post-Texian uplift, succeeding the Post-Fernandian, has strong 

 expression in a part of Mason County. West of Fly Gap and Herman Creek 

 it is especially prominent. Here there is a development of a remarkable iron 

 ore upon a large scale. This stratum, lying in the normal north-south trend 

 of the Texian System, is a most extraordinary aggregation of Burnetian, Fer- 

 nandian, and Texian mineral components, commingling as it does the rare 

 elements of the first named, the magnetite of the Fernandian, and the pecu- 

 liar quartz rock of the last named period. Narrow bands of the Texian 

 quartzite alternate in the same mass with smooth, glistening layers of the iron 

 ore, which carries considerable titanium and rare elements such as tungsten 

 and niobium. With these are also ribands of a scaly specular hematite sand- 

 stone. 



c. The great Silurian geanticline, which further modifies the structure and 

 complicates the mineral relations in some areas, was chiefly manifested in the 

 Archean and Algonkian tracts by vein fissures, now filled by quartz which 

 often carries alternate varieties of iron (and perhaps manganese) ores similar 

 in composition to that mentioned under b, but usually without the rarer in- 

 gredients. 



46— geol, 



