532 PHYSICAL GEOGRAPHY. 



carefully explored. Meanwhile I would bespeak the attention of my 

 brethren of the hammer to the brief statement of my theory in this 

 chapter, to be followed by fuller technical descriptions in the next vol- 

 ume. If confirmed by further research, these studies will throw much 

 light upon the origin of granite, and may indicate the existence of the 

 Labrador system in many localities where nothing of the kind is now 

 suspected. 



THE HURONIAN AGE. 



As far back as 1822 my father distinguished this formation in Massa- 

 chusetts from all the other crystalline schists by the names of talcose 

 and chlorite slate.* Essentially the same delineation has since appeared 

 on all the geological maps of Massachusetts. The Vermont geological 

 map shows its continuation through that state. In 1857, Prof. H. D. 

 Rogers proposed to distinguish this formation farther south, in Penn- 

 sylvania and Maryland, by the name of Azoic, separating it from the 

 underlying Hypozoic gneisses. He regarded the group as older than 

 his "Primal series." In 1858,! I understood him to express his belief in 

 the equivalency of the Azoic with the formation called Azoic system by 

 Foster and Whitney in Michigan (1850), as separated from their "igneous 

 granite," and with the Huronian system of Logan, described in 1855. 

 Singularly enough, while Prof. Rogers perceived the true position of the 

 Pennsylvania rocks, he believed the New England talcose series to be 

 metamorphic Paleozoic. James Macfarlane seems to have been the first 

 to express the opinion that the Canadian and New England area was of 

 Huronian age. In this opinion he was speedily followed by Prof. Her- 

 mann Credner in 1869, and by Dr. T. Sterry Hunt in 1871. In my first 

 annual report I accepted Logan's interpretation of these rocks, calling 

 them altered Quebec, and belonging to the Lower Silurian, but since 

 1871 have thought it better to refer them to the Huronian. 



The Huronian rocks, as seen by the fourth of our maps, have not 

 played a very important part in our history, judging from the small areas 

 now occupied by them. But the amount of time occupied in their depo- 

 sition, alteration, and elevation much exceeded the length of the previous 

 eon. 



* Amer. your. Set., I vol. vi, p. 26. f See Proc. Boston Soc. Nat. Hist., vol. xv, p. 306, for further references. 



