■ 



1858.] BIGSBY PALEOZOIC ROCKS, NEW YORK. 305 



From the fact that the atoms of protoxides exceed the atoms of per- 

 oxides, it appears that this elvan cannot be composed of quartz, 

 felspar, and green mica. 



If we suppose that hornblende replaces mica in this elvan, and 

 that half the iron is present as protoxide, and that all the alkalies 

 belong to the felspar of the rock, it is easy to calculate the fol- 

 lowing 



Miner alogical Composition of the Soda-Elvan. 



Quartz =29*52 



Felspar ..=60*15 

 Hornblende = 8*81 



98*48 



From the discussion of the granites of the potash- and soda-type 

 in this district, I feel disposed to draw the following conclusions, 

 which are confirmed in a remarkable manner by some observations 

 I have recently made, in conjunction with Mr. Jukes, in the County 

 Wicklow : — 



1st. That both in Leinster and the county Down, the potash-granites 

 are more constant in composition, both mineralogical and chemical, 

 than the corresponding soda-granites. 



2ndly. That the potash-granite appears to be the standard type of 

 granite, from which other granites and crystalline rocks are formed 

 by the addition of bases ; for example, the anorthite syenite of Car- 

 lingford, and the soda-granites of Newry, and in Leinster the out- 

 lying patches of granite between the main chain and the sea. 



3rdly. That the potash-granite of Leinster is more persistent in 

 external character than the potash-granites of Newry, although the 

 latter are equally constant in chemical composition. 



3. On the Palaeozoic Rocks and Fossils of the State of New 

 York. Part II. The Stratigraphy and Classification of the 

 Series. By J. J. Bigsby, M.D., F.G.S. 



[The publication of this Paper is deferred.] 



(Abstract.) 



In the synoptical view of the strata and fossils of the palaeozoic basin 

 of New York, read before the Society Nov. 18, 1857, the author 

 desired to arrange the vast stores of information contained in the 

 Official Reports of the State Geologists of New York in a metho- 

 dical and accessible form ; and in this Second Part of the Memoir 

 he treated succinctly of the stratigraphical arrangements hitherto 

 used, and the classification now adopted by himself. This is but 

 little modified from that proposed by De Verneuil ; and is mainly 

 characterized by the union of certain sections of the series into 

 natural groups, and by the establishment of a distinct middle Silu- 



