ARCHEAN GROUP. 559 



I. ARCHEAN GROUP. 



The Archean rocks in this portion of Texas are those which make up the 

 greater part of Llano County and the eastern half of Mason County, extend- 

 ing also into western Burnet County and the adjoining portions of Blanco 

 and Gillespie counties on their north line. The rocks are essentially crystal- 

 line, and within the belts which they occupy are to be sought most of the 

 rare minerals and valuable metallic ores of the district. There are two well 

 marked systems which have such pronounced features as seem to entitle them 

 to be placed as the probable equivalents of the Laurentian and Ontarian sys- 

 tems, respectively, of British America and the Northwestern United States. 

 Our strata are apparently more nearly related to those exposed in the region 

 of Lake Superior and the Lake of the Woods. 



1. BURNETIAN (LAURENTJAN) SYSTEM. 



The oldest rocks are gneisses, gneissic granites, and schists, which, in most 

 exposures, trend north 75° west, or within 6° of an east- west line by compass 

 bearings. They usually dip at high angles and can often be traced readily 

 for long distances be repetitions of their peculiar exposures. In places where 

 later trends have partially cut them out they may sometimes be turned into 

 different courses, but this complication is only occasional, granitic and feld- 

 spathic dikes of later date having confused the structure in some places, how- 

 ever. Mounds of white quartz, often carrying black and greenish micas in 

 large plates, are especially characteristic, and in these the rare minerals are 

 apt to be found. Belts of garnet deposits are another feature, and the tin 

 ore recently discovered by the writer occurs in this system. Masses of feld- 

 spar and graphic granite and a great variety of interesting minerals abound 

 over the areas in which these rocks are uncovered. A separation into series 

 has been attempted, although it may be doubted if we have knowledge enough 

 to make distinctions which will stand the test of the most thorough study. 



1. LONE GROVE SERIES. 



The basal gneisses and gneissic granites, or granitoid gneisses, with the 

 graphic granites and some felsite porphyry carrying the belts of rare min- 

 erals, are now regarded as a separate series, which is fairly exemplified in the 

 vicinity of Lone Grove. This terrane crosses the Central Mineral Region 

 probably in three or more axes, and it is especially prominent in the eastern 

 half of Llano County and over a limited area in western Burnet County, al- 

 though it extends well into Mason County. The search for tin ore should be 

 conducted along these lines. 



