862 J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



be positively stated in regard to the Glastonbury uraninites is that they 

 are upper Paleozoic. Granitic intrusions occurred repeatedly in New 

 England and appear to Imve covered quite a range in time, since Carbon- 

 iferous formations rest in places upon somewhat older granites as well 

 as being cut by those of younger age. 



Middle Precamhrian age of the Llano series. — As to the age of the peg- 

 matites in the Llano series in Texas, Becker bases his statement that they 

 are "not far below the Cambrian'' on a brief observation by Walcott, made 

 24 3'ears previously, on what the latter guardedly calls a "hurried recon- 

 naissance of a portion of the I*aleozoic area of Central Texas, the chief 

 object in view being the study of the Cambrian section and the collecting 

 of fossils from the Texas Potsdam horizon."^^^ The rocks which lay un- 

 conformably below the Upper Cambrian (Potsdam) Walcott named the 

 Llano group and correlated them with the Grand Canyon group, which at 

 that early time he placed in the Paleozoic, but later removed to the Pre- 

 cambrian. Walcott notes that on Eoesslers map of Llano County (1875) 

 all of the Llano group is referred to as "granitic, metamorphic, and 

 igneous" and has been considered as Archean. He, however, states that 

 he did not observe any rocks of undoubted Archean age. 



In the Llano-Burnet folio, Texas,^"^^ this region is mapped and its 

 geology discussed in detail. Paige gives the following in regard to the 

 character and age of the Llano series :^"'^ 



"The Llano series has proved to be a completely metamorphosed series of 

 schists, marbles, and gneisses and can be classified as Algonkian in contradis- 

 tinction to Archean only on the very broadest evidence, such as the prepon- 

 derance of metamorphic sediments over igneous material. Roessler's descrip- 

 tion of these rocks as granitic, metamorphic, and igneous was therefore concise 

 and, possibly except for his reference of the series to the Archean, correct." 



The granites intrude the Llano series in great volume and the uranium 

 minerals are in pegmatites related to the granites. Thus the age is dis- 

 tinctly Precambrian. The geological relations do not prove more, but the 

 general character is similar to the intrusive granites of the middle Pre- 

 cambrian of Canada, the Huronian or post-Huronian of some authors, 

 the late Archean as defined by Lawson. These are the Lorrain, or Algo- 

 man granites, as they have been variously named. Thus there is no such 

 closeness of age as Becker concluded to exist between the Glastonbury 

 (Portland) uraninites and the radioactive minerals of the Llano series. 



118 C. D. Walcott : Note on Paleozoic rocks of central Texas. Am. .Jour. Sci., vol. xxiii. 

 1884, pp. 4.31-433. 



119 Sidney Paige : Geologic atlas of the United States, No. 183, U. S. Geol. Survey, 1912. 



120 Loc. cit., p. 2. 



