430 PROCEEDINGS OF OTTAWA MEETING. 



amygdules arc found of all sizes down to ordinary ones and with all gradations in 

 color into the white chalcedony and quartz. As the chert and jasper formation in 

 one oft lie districts rests directly upon the amygdaloid it is believed that the jasper 

 of the ore formation and that in the amygdaloid are secondary rocks, formed 

 simultaneously, as has been advocated by Irving and myself in reference to the 

 major pari of the ore formations of the Lake Superior region. 



The term greenstone-conglomerate, rather than agglomerate, is used because this 

 latter implies a definite theory of origin. A study of these rucks, in the field and 

 under the microscope, shows undoubted gradations from rocks which are true 

 detritals to those which are tnti's, from this to tuff and lava intermingled, and 

 then to tine lava Hows, which are often brecciated. Probably also some of the 

 rocks are true agglomerates, some of the elastics are undoubtedly sub-aqueous 

 ash-beds, from which there arc gradations into the true detritals. 



The interstratification of volcanics and detritals, combined with the lithologic 

 similarity of the rocks, at once suggests a comparison between the Huronian vol- 

 canic series and tin' Keweenawan. There are, however, important differences. 

 The Huronian volcanics are for the most part Aery much more altered than those 

 of the Keweenawan. Many of thorn have been sheared, and they then pass into 

 crystalline greenstone-schists. The detritals, in common with the igneous rocks, 

 have also frequently been metamorphosed into mica-schists, hornblende-schists, etc. 



These volcanic seriesare known at various places south of Lake Superior, hut the 

 most extensive areas are at the east end of the Gogebic series, west of Gogebic lake, 

 Michigan, and north of Crystal Fall, in the Michigamme district, Michigan, in 

 the < rOgebic area the volcanic -roup is 7,000 or 8,000 feet in thickness. The strata. 

 in approaching the volcanic center, both from the east and the west, take a sudden 

 swing to the south, showing a sinking of the formations about the volcanic foci, as 

 a result of the loading of the earth's strata by a mountainous mass of volcanic ma- 

 terial. Moreover, this great thickness of volcanic material stands as the equiv- 

 alent in time to the iron-bearing formation of the west, which is, upon an 

 average, not more than 700 or 800 feet thick. "We thus are able to compare in 

 this area the rate of deposition of a formation analogous to a limestone and a 

 group of volcanic strata. Also the district gives an excellent illustration of the 

 principle that in the pre-Cambrian rocks, lithologic character may be no guide as 

 to age, for within a few miles we have a simple sedimentary series passing laterally 

 into a volcanic series. If the two areas chanced not to be connected, there would 

 ajreal temptation to infer that they are not contemporaneous. 



Remarks upon Professor Van Hises paper were made by W. H. ( '. 

 Smith and W -I McGee. 



In the absence of the author of the following paper, it was read by 

 Mr W -I McGee: 



ON TWO OVERTHRUSTS IX EASTERN NEW YORK* 

 BY N, II. DARTON 



l.ast autumn I mapped the Helderberg and associated formations in New York 

 for the geologic map of that state now in course of publication. In the western 

 and central portions of the state these formations lie in the general, gently south- 



* Extracted by permission from a report to Dr James Hall, State I logisl of New York. 



