AGE OF THE LLANO SERIES 8G1 



Now these feldspar dikes of Portland are intruded into the Bolton 

 schist, and so also are nearly all of those in the town of Glastonbnry.^^^ 

 All the feldspar deposits and their uranium minerals come from peg- 

 matites. As no fossils have been found in the metamorphic rocks of Con- 

 necticut, the age relations are in many places obscure. Nevertheless Rice 

 and Gregory consider that the relations to the rocks of eastern Massa- 

 chusetts suggest a late Paleozoic age for most of the schists east of the 

 Connecticut Eiver, though they do not exclude the possibility of an earlier 

 Paleozoic age. These authors do not express any opinion as to the age 

 of the Glastonbury gneiss, but state that it is "of uncertain origin."^^* 

 There is no evidence, however, that it is Cambrian. The placing of the 

 Wilbraham gneiss, the Massachusetts equivalent of it, in the Cambrian 

 goes bax3k to a decade when the banded or mashed granite-gneisses were 

 generally believed by the students of New England geology to be of sedi- 

 mentary origin. Part of them were regarded as Archean, another part 

 as basal Cambrian. As Van Hise explains, the placing of a great series 

 of granite-gneisses in the basal Cambrian was a natural error, owing to 

 the intricacy of the structure and the fact that the Cambrian has at its 

 base an arkose formation which in places is quite similar in appearance 

 to the granulated granites from which they were in part derived. ^^'^ Be- 

 fore 1908, however, Emerson had recognized that much of what was 

 formerly regarded as sedimentary gneiss of Cambrian age in western 

 Massachusetts was in reality an Archean basement of igneous origin. In 

 1898 Emerson placed the Belchertown tonalite and Williamsburg granite, 

 covering large areas near the Connecticut Valley, in the Carboniferous.^^^ 

 The map showing these formations did not include, however, the eastern 

 upland. In 1916, in a preliminary geologic map of Massachusetts and 

 Rhode Island,""^ Emerson assigns a late Carboniferous or post-Carbon- 

 iferous age to the Monson granodiorite, which is the Massachusetts ex- 

 tension of the Connecticut Glastonbury gneiss. Thus there has not been 

 at any time a sufficient basis for assigning a Cambrian age to the Glaston- 

 bury or Portland uraninites. The evidence of the late or post-Carbonif- 

 erous age of these granites is derived from a region farther east in Massa- 

 chusetts, where granites cut schists in which Carboniferous plant fossils 

 have been found. It appears to the writer that all which can at present 



"3E. S. Bastin : Economic geology of the feldspar deposits of the United States. Bull. 

 420, pi. vi, U. S. Geol. Survey, 1910. 



"* Rice and H, E. Gregory: Manual of the Geology of Connecticut, p. 114, Bull. 



No. 6, Conn. Geol. and Nat. Hist. Survey, 1006. 



115 C. R, Van Hise and C. K. Leith : Precambrian geology of North America. Bull. 360, 

 U. S. Geol. Survey, 1009, pp. .587-592. 



ii« B. K. Emerson : Holyoke folio, TJ. S. Geol. Survey, 1808, 



1^7 U. S. Geol, Survey. To be published In 1917. 



