From the Proceedings of the American Association for the Advancement of Science, Portland 



Meeting, August, 1873. 



THE M1GARAAND LOWER HELDERBERG GROUPS: 



THEIE RELATIONS, AND GEOGRAPHICAL DISTRIBUTION 

 IN THE UNITED STATES AND CANADA. 



By JAMES HALL. 



In proceeding to the discussion of this subject, I propose in 

 the first place to cite a paper read by Mr. A. H. Worthen at 

 the Troy Meeting of the American Association, and published 

 in the Proceedings under the following title : 



" Remarks on the Relative Age of 'the Niagara and so-called 

 Lower Helderoerg Groups. By A. H. Worthen, of Spring- 

 field, Illinois" 



" Recent investigations have developed certain facts, bear- 

 ing upon the question of the relative age of the above-named 

 groups, which we desire to present in a brief manner for the 

 consideration of those who are especially interested in strati- 

 graphical geology. 



" In northern and western Illinois, from the mouth of the 

 Illinois river northward to the Wisconsin line, the Upper Silu- 

 rian division of the palaeozoic series is represented by buff, 

 gray or yellowish-gray dolomites, sometimes in remarkably 

 even beds, as at Joliet and Grafton ; and at other localities by 

 concretionary masses, with but faint traces of stratification, as 

 at Bridgeport, near Chicago, and at Port Byron and Leclaire, at 

 the head of the Upper Rapids on the Mississippi river. They 

 range in thickness from seventy-five to three hundred feet, 

 and directly overlie the shales and argillaceous limestones of 

 the Cincinnati group of the Lower Silurian series. These dolo- 

 mites are quite fossiliferous, and afford many characteristic 

 Niagara species, among which we may mention Pentamerus 

 oblongus, Spirifer radiatus, Calymene Blwmsnoachii, Cary- 

 ocrinus ornatus, Orthoceras undulatum, etc. From the 

 Bridgeport locality alone, nearly one hundred species of fos- 

 sils have been enumerated, a large number of which are spe- 

 cifically identical with those found in the Niagara beds of New 

 York and Canada ; and, so far as we are aware, all western 



