308 On the Constituents of Primary Rocks, 



This fossil fucus is readily recognized as a species allied to the 

 F. alleghaniensis, which I recently described m the Journal of the 

 Academy of Natural Science, vol. vi. from which it diifers prin- 

 cipally in the elongation and uniformity of the stem, its sub-quad- 

 rangular form, in general, and in being more compressed and 

 elevated on the surface of the stone. The branches of the pre- 

 sent species are less fastigiated, and more remote from each other: 

 in no instance are the tops of the branches exposed to view in 

 the specimens which have come under my cognizance. The 

 largest stem is one third less in its greatest visible diameter in 

 the present species, and they intercept, cross, or run into each 

 other in various directions, so as occasionally to assume an ap- 

 pearance not unlike the asterias. 



I have seen a very perfect specimen of this fossil, from the vi- 

 cinity of Lockport, N. Y. in the possession of W. R. Johnson, Esq. ; 

 and Mr. Peale's museum of N. York possesses a very large slab 

 of these fossils. I am indebted to the politeness of P. A. Brown, 

 Esq., for the opportunity of describing this species, who obtained 

 it in the state of New York, during a geological excursion last 

 summer. Specimens in the cabinet of the Academy of Natural 

 Science, cabinet of Mr. P. A. Brown, &c. 



In consideration of the great obligation under which Dr. 

 Brongniart has placed all admirers of oryctology, by the publica- 

 tion of his invaluable " Vegetaux fossiles,^^ I have taken the liberty 

 to designate this species by his name. 



GENERAL REMARKS ON THE CONSTITUENTS OF 

 PRIMARY ROCKS. 



We have on this continent a very extensive geological limit, 

 constituted of primary rocks and their subordinates. It consti- 

 tutes an inflected Hne, commencing in the north, and passing 

 southwardly from the indented shores of Maine, New Hampshire, 

 Massachusetts, Rhode Island, and Connecticut, to the city of 

 New York, of which it forms the base. Thus far, this limit is 

 bounded by the ocean, and has for its general mineralogical cha- 

 racter, the rocks commonly called granite and gneiss. There 

 are many varieties of these two rocks, occasioned by the varying 

 proportions in which their respective constituents are found 

 together. Granite has for its constituents, felspar, quartz, and 



