14 TRANSACTIONS OF THE [OCT. 19, 



erlv one described in this paper is in Mt. Eoliis, in the town of Dor- 

 set, Yt.' F. L. Nason has mentioned others from Mt. Holly.^ We 

 have previously described several from the vicinity of Whitehall 

 and Rutland.' ' North of this point many dikes are recorded in the 

 Yermont Reports and in the earlier work of Zadock Thompson,* 

 hearlv all of which have been revisited and studied in their sections. 

 On the New York side, the most southerly recorded dike is near 

 Glen's Falls,^ and several others have been discovered by F. L. Na- 

 son near the town of North River, specimens of which have been 

 very kindly placed in our hands. Further north in the region of 

 the magnelite mines, hardly an excavation has been made which 

 does not reveal them. They occur at Hammondville, Mineville, 

 Palmer Hill, at the Arnold mines, and at Lyon Mountain. Many 

 of these are mentioned in volume xv of the Tenth Census, in B, T. 

 Putnam's report on the iron ores. Many other dikes are revealed 

 by the cuts of the D. & H. R. R., especially near Port Kent. The 

 great (supposed) dike at Avalanche Lake, in the Adirondacks, was 

 early recorded," but is shown by our sections to be a shear zone, 

 Avhich will be elsewhere described. 



From this brief outline it will be seen that eruptive action has 

 been widespread in the area treated. Similar phenomena are likewise 

 known across the national boundary as far as Montreal, where they 

 are the most extensively developed of any district in the entire 

 region. 



The dikes are of two greatly contrasted kinds of rock. The one, 

 quite acidic and essentially feldspathic, is related to the porphyries, 

 trachytes, and keratophyrs ; the other, much more basic, includes 

 diabases, camptonites, and monchiquites. The former corresponds 

 to the dike rock bostonite, lately proposed by Rosenbusch as a name 

 for the trachytic dikes, which are well nigh universally associated 

 with elaeolite syenite, and this name will be here employed. We 

 feel conservative about the introduction of new names into our 

 already overburdened science, the more so, as in this case the rocks, 

 do not essentially differ from effusive trachytes, but as they occur in 

 dikes and far from any extended central outbreak, and as the insti- 

 tution of the dike rocks affords so much convenience in correlation, 

 the special term is adopted. 



The Bostonite {Poyyhyt'y, Trachyte) Dikes. — The name boston- 



1 Geology of Vermont, vol. ii, p. 586. 



2 F. L. Nason. A New Locality for the Camptonite of Hawes and Rosen- 

 busch. A. J. S. iii, xxxviii, 229. 



3 J. F. Kemp and V. F. Marsters. Camptonite Dikes near Whitehall, N. Y. 

 Amer. Geol., iv, 97. 



* Zadock Thompson. Appendix to Thompson's History of Vermont, Bur- 

 lington, 1853, p. 53. 



5 E. Eminons. Geol. of 2d Dist. N. Y. State Survey, p. 184. 



6 W. C. Redfield. Some Account of two Visits to the Mountains of Essex 

 County, N. Y., 1830-37, etc., Amer. Jour. Sci., i, xxxiii, 301. See also First 

 Ann. Rep. N. Y. State Survey, 1837, p. 131 ; 2d do., 1838, p. 223; and Em- 

 mons' final Report, p. 215. 



