398 NEW YORK STATE MUSEUM 



tion of them. Ttiey occur for the most part in very irregular 

 dikes, often of very small width. They belong to the peridotite 

 class of dgneousi rocks, the most basic of any. Such rocks are 

 prone to rapid decay, and these are no exception, all of the 

 known exposures being considerably, and many of them highly 

 altered and rotted. 



In general their mineralogic make-up is of biotite, olivin, py- 

 roxene and melilite, with accessory magnetite, perovskite and 

 apatite. Wherever sufficiently fresh material has been forth- 

 coming, the presence of melilite has been noted, Smyth having 

 early shown its presence in the Manheim rock and having 

 recentl}^ detected it in some new material from the Syracuse 

 viciinity. Alnoite is the name applied to a melilite holding 

 peridotite. All the rock contains biotite and olivin in quantity, 

 but the pyroxene is much more irregular in its occurrence. It 

 is rare or else absent in the Manheim rock; while Kemp has 

 shown that it is the main mineral of the ground-mass in the 

 Dewitt dike near Syracuse. Likely some glassy base was 

 present in nearly all occurrences. In all except the freshest 

 rocks the olivin has gone to serpentine, and Smyth has described 

 in detail the processes of alteration and decay. 



Chemical analyses. Since these rocks are all considerably 

 altered, analyses of them are not trustworthy if what is sought 

 is the actual composition of the fresh rock. Yet, if the analyses 

 are made from the freshest possible material they will give good- 

 evidence of the general character of the igneous magma, and of 

 the closeness of correspondence of the rocks from the separate 



occurrences. 



12 3 4 



SiO, 36.8 33.8 35.25 37.44 



Al.,03 4.16 6.84 6.1 28.6 



FeoOa n.d. 12.26 8.53 11.92 



FeO 8.33 n.d. 5.6 n.d. 



MgO 25.98 21.38 20.4 1.97 



CaO 8.63 9.5 7.4 5.45 



Na.O .17 n.d. .7 .97 



KoO 2.48 n. d. 2.88 1.02 



Loss^ 12.25 15.2 12.4 12.67 



Sum 100.27 98.98 99.26 100.04 



'In no. 1 includes H„0 -1-6.93, 1L0-..51, CO. 2.95, TiO, 1.26, P0O5 .47, 

 MnO .13; in no. 3 includes TiO, 2.25. 



