130 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



which are essentially alike. Professor Dana considered these rocks as 

 composed of pyroxene, magnetite, and labradorite. 



Professor Howe^ soon aiterward made further determinations of spe- 

 cific gravity with the view of illustrating more fully the uniformity of 

 this composition. 



Chemical analyses of these rocks have been made by Prof. G. H. Cooke,^ 

 Prof. W. G. Mixter,3 S. T. Tyson,^ and Dr. F. A. Genth,^ which are all 

 nearly concordant, and show the ultimate composition to be always 

 nearly the same. 



The most extensive series of analyses has been made by myselP upon 

 specijnens taken from various points in the Connecticut Valley, and a 

 specimen from Jersey City, which was intended to represent the Hud- 

 son palisades. These analyses demonstrated the essential uniformity of 

 the composition, all variations being referable to the degree of hydra- 

 tion which represents the extent of the decomposition of the rocks, and 

 the degree of alteration of its pyroxene to chlorite. I concluded that 

 the feklspar was labradorite, but demonstrated that the large kernels in 

 one variety were of anorthite. 



Prof. E. S. Dana'' began the microscopic examination of these rocks. 

 He confirmed the determination that these rocks are composed of augite, 

 triclinic feldspar, and an iron oxide, and assumed from my analysis 

 that this feldspar was labradorite. Notwithstanding this, these rocks 

 are stated to this day by Credner to be diorites, that is, hornblendic 

 rocks. 



Mr. P. Frazer^ has discussed my analysis of West Eock together with 

 the analysis by Professor Genth. He assumed the feldspar to be a 

 labradorite of normal composition, and calculated that labradorite and 

 augite were present in equal proportion. 



From these works it is then evident that the unaltered Mesozoic dia- 

 bases are all very much alike, and are composed of augite, iron oxide, in 

 the form of magnetite and titanic iron, and a feldspar that has been 

 reasoned to be labradorite.^ This latter determination is in need of 

 verification. 



The method employed by me for this determination was that proposed 



1 Philosophical Magazine, February, 1876. 



2 Geological Report, New Jersey, page 215. 



3 American Journal of Science, iii, vol. vi, page 105. 



*2d Geological Survey of Penna. report of progress in York and Adams counties; 

 page 120. 



s American Journal of Science, 1875, vol. is, page 185. 



^Proc. Am. Association Adv. Sci., Aug., 1874. 



•^ Credner, Gdologie, page 532. With his ill-defined definition of melaphyr, to which 

 he refers a part of these rocks, they certainly have nothingto do. These rocks had 

 long been demonstrated to be augitic when the last edition of his work appeared, 

 which still states the palisades to be of diorite. 



8 2d Geological Survey of Penna., vol. C, report of progress in the district of York 

 and Adams counties. 



9 1 speak now only of its essential original constituents. Biotite and hornblende 



