454 



THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



In a criticism on the Chemical and Geological Essays of Dr. Hunt, 

 Professor Dana further states (Am. Jour. Sci., 1875, (3) IX., pp. -102, 

 103): 



" The reader of the volume will observe that in the Thifd Chapter the 

 White Mountain series and Green Mountain series of rocks are made (as had 

 been done by other geologists) Lower Silurian, and Upper Silurian and Devo- 

 nian, in age, while in the Thirteenth Chapter (as also mentioned in the preface 

 to Chapter III) both are pronounced prc-Silurian, . In this, the older view, as 

 I believe I have proved, is the one sustained by the facts. The new view is 

 wholly speculative, being based on no careful stratigraphical study of the 

 regions, but mainly upon the assumption that certain kinds of crystalline 

 rocks arc a test of geological age the world over. Since the first announcement 

 of this doctrine by Mr. Hunt, I haVe spent many months in the study of the 

 Green Mountain rocks and those of some other parts of New England, in order 

 to ascertain whether there is any virtue in the criterion ; and I have found 

 none. Mr. Hunt makes stanrolite evidence of pre-Silurian age ; while, as I 

 have shown, its crystals occur in crystalline rocks of New England that are 

 not older than Upper Silurian. Such erroneous conclusions make it apparent 

 that in reading the work the judgment should be held in reserve until the 

 other side is heard. There is also another more serious reason for this reserve. 

 For the volume contains a series of misrepresentations of the views of others 

 wholly unnecessary to the presentation of the author's opinions, and difficult 

 to find excuse for." 



Professor Dana, in 1873, after discussing the observations of himself 

 and others, remarks : — 



" From the facts which have been presented it follows that all old-looking 

 GreeTi Mountain gneisses are not proe-silurian, and, further, that the presence 



of staurolite^is no evidence of a praj-silurian age It is not easy to avoid 



the conclusion that the Taconic slates are Hudson river slates The Tren- 

 ton limestone and Hudson River or Cincinnati groups, which properly con- 

 stitute one series in American Geological History, are then the true Taconic 

 system^' (Proc Am. Assoc. Adv. Sci,, 1873, XXII., B., pp. 25-29 ; Ameri- 

 can Naturalist, 1873, VII., pp. 058-CGO). 



Professor Dana, in giving an account of an examination of the Hel- 

 derberg rocks in the valley of the Connecticut (Am. Jour. Sci., 1873,. 

 (3) VI., p. 348), concludes that stanrolite and andalusitc occur in meta- 

 morphic rocks of any ago, remarking that 



*' this Helderberg series in Central New England comprises a large part of the 

 connnon kinds of inetaniorphic rocks, gneiss of several varieties, undistin- 

 guishable lithoiogically from the oldest ; hornblende rock and schist ; syenite 

 gneiss ; coarse mica schist and mica slate ; ataurolitic slate. 



" A large nart of the rocks that have been distinguished as of the ' Mont- 



