PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 129 



with a. few spiny tubercles, none of them more than one-fourth as long- 

 as the eye. The specimen sent is a dried individual and its colors cannot 

 be made out. There is a small slit behind the fourth gill. The local 

 name at York Factorj^ is " Miller's Thumb," according to Dr. Bell. 



27781. CoREGONUS Artedi LeS. var. 



The species agrees in all respects with typical Artedi from the Great 

 Lakes with the exception of its smaller eye. Two larger examples col- 

 lected at Moose Factory many years ago also have the eye notably 

 smaller than in C. Artedi. The local name at York Factory, says Dr. 

 Bell, is " tulihV^ It must not be inferred, however, that the species is 

 at all like Coregomis tulUbee, for it is not closely related to this form. 

 Specimens in alcohol are much desired. 



U. S. National Museum, 



Washington, D. C, March 28, 1881. 



OIV THE MHVERAI^OOICAL. COMPOSSTIOIV OF THE: IVOKITIAr. ITIEMO* 



Z05C diabase: upoiv the: ati>aivti€ isorde:r. 



By GEORGE 'W. HALVES, Ph. I>. 



In my opinion the Mesozoic " trap rocks" have excited more interest 

 and received more lithological attention than any other defined rock 

 species upon the Atlantic border. There is, therefore, no rock concern- 

 ing the geological features and chemical comi)osition of which we are so 

 well informed; but much as it has been discussed, the mineralogical 

 composition has, in part, remained a matter of speculation rather than 

 of definite knowledge. As our methods for determining such points are 

 now much more satisfactory, I think that the final determination of the 

 mineral comi)osition of the normal variety of this rock may be accom- 

 plished, and this will be of much interest on account of the wide dis- 

 tribution and the uniform character of these diabases. 



1 will give a few references to show the development of our knowledge 

 of these rocks and the essential uniformity in their composition. 



When Perceval wrote, no attempt was made to determine their com- 

 position, and their geological features and diytribution were chi-efly con- 

 sidered.^ 



Prof. J. D. Dana has at different times pointed out the wonderful 

 uniformity of these rocks wherever they occur, intersecting the Meso- 

 zoic sandstones on the Atlantic border.^ He quotes specific gra\'ity 

 determinations by Professor Brush of Is'ew Haven, Professor Cooke of 

 New Jersey, Professor Kerr of Ealeigh, N. C, and Professor Howe o£ 

 Nova Scotia, made uj)on specimens from their respev^tive localities, and 



1 See Dana's Geology, page 20. 



^Americau Journal of Science, series iii, vol. vi, page 104. 



Proc. Nat. IVIus. 81 9 June 2S, 1 8i8 1. 



