65 JOtJRNAt OP THi: 



ionally in the Alg*onkian. They mig-ht appropriately be 

 named the Monroe slates. 



The Volcanic Series. The middle member of Emmons' 

 Lower Taconic is the quartz rock (white and brown sand- 

 stone), which is stated by him to exist in many varieties; 

 (p. 57). I did not observe any true g-ranular quartzites, 

 such as characterize the Cambrian for instance (even in a 

 metamorphosed state); and in fact, from Emmons' and 

 Kerr's descriptions, their rocks of this class are rather 

 fine-g*rained, thin bedded, quartzose schists, and devitri- 

 fied quartz or chert, often porphyrized and brecciated. 

 It is probable that Emmons' fine sfrained talcose quartz 

 (p. 58) corresponds to the silicified schists, (p. 63) whose 

 quartzitic nature is due to a later hydro-silicification. 



The crypto-crystalline varieties of quartz (flint, chert, 

 hornstone) are of especial interest, and warrant a careful 

 consideration. It is at present the opinion that these 

 rocks belong- to the class of ancient (pre-Cambrian) acid 

 volcanics, in many respects analog^ous to, and probably 

 contemporaneous with, similar rocks of the South Moun- 

 tain in Maryland and Pennsylvania, whose discovery was 

 first announced by the late Dr. Georg-e H. Williams, i 

 Miss Florence Bascom has described the orig-in, devitri- 

 fication and structure of the acid types of these rocks. *^ 

 And Dr. Williams has outlined the g-eneral distribution 

 of the ancient volcanic rocks along" the eastern border of 

 North America.'* These rocks are analog-ous also to the 

 halleflintas and eurites of Southern Sweden, described as 

 volcanic rocks by Nordenskjold. They would also cor- 

 respond to Hunt's pre-Cambrian petro-silex rocks, called 

 by him the Arvonian, being" below his Huronian. 



The hornstones have every appearance of being* acid 

 feldspar quartz rocks, and will probably be found, on 



1 The Volcanic Rocks of the South Mountain in Pa. and Md. Ani. 

 Jour. Sci. xliv., Dec. 1892. pp. 482—496. 



2 Jour. Geology, Vol. 1. 1893. pp. 813—832. 



3 Jour. Geology, Vol. 2. 1894. pp. 1—31. 



