ARCHAEAN GROUP. 259 



A reasonable explanation of the slower erosion of these elevated remnants 

 may be found in the fact that they are made up of harder or more durable 

 material than the plateau gneisses; and they also occupy positions in the sum- 

 mits of folds, which have apparently been the escape valves for igneous in- 

 trusions of later date. 



(3.) THE CROWNED PEAKS. 



Most of the exposures of granitic rocks in direct contact with the Potsdam 

 Sandstone and later strata are of different character from the Burnetan gneisses, 

 and their trends and their relations to the overlying beds show that their 

 eruption has been later than the deposition of the capping material. These 

 occur along the borders of the area of the exposed Archaean rocks, and they 

 have all been elevated very materially above the general level of the sedi- 

 ments with which the cappings were originally continuous. I have not ob- 

 served a single case of Post-Archaean sediments resting upon such granitic 

 knobs, without being more or less affected in dip by the underlying protru- 

 sions. Moreover, in all such cases the evidence of faulting upon one side (or 

 more) of such remnants is very pronounced. 



The rocks of the Burnetan System are largely gneisses, but they graduate 

 upon the one hand into quartzose mica schists, and upon the other into fria- 

 ble sandy gneisses and fine-grained binary granites and graphic granite. 

 There is considerable variety in color between the almost white micro-granu- 

 lar saccharoidal aggregations of quartz grains with few minute mica scales, 

 ana the green and black and bronzy chlorite, amphibole, and pyroxene schists. 

 Quartz, as an ingredient of most of the rocks, and as dikes or beds coincident 

 with the regular strike, is very prominent. Orthoclase is by far the most 

 abundant feldspar, although albite is not uncommon in parts of the system. 

 Crystalline labradorite occurs in a bed exposed in Hoover Valley, Burnet 

 County, but it has not been recognized elsewhere. Felsitic rocks are preva- 

 lent in some parts of the system. An interesting porphyritic orthoclase fel- 

 site, from Long Mountain, Llano County, carries much blue opal in scattered 

 grains of the size of morning glory seed. 



A large nubmer of minerals, including new and valuable species, have 

 been taken from this system in Burnet and Llano counties. Among them 

 may be named actinolite, albite, almandite, andesite, andradite, amazon-stone, as- 

 bestus, beryl, biotite, bronzite, calcite, carnat, cassiterite{f), chlorite, chloropal, cyrt- 

 olite, epidote, fassaite, fergusonite, fibrolite, fluorite, gadolinite, graphite, grossularite, 

 gummite, hematite, hyalite, hypersthene, idocrase, ilmenite, jefferisite, heilhauite, lab- 

 radorite, magnetite, marcasite, margarodite, martite, microcline, muscovite, molyb- 

 denite, molybdite, opal, orthoclase, pyrite, pyrolusite, psilomelane, pyroxene, quartz, 

 serpentine, talc, tengerite, tourmaline, and the new minerals nivenite, thorogum- 



