74 ^'t:w york state museum 



h e m i p 1 i c a t a zone of T. G. White, (20 feet 9 inches), and 

 this in turn is overlain by beds characterized by T r i n u c 1 e u s 

 concentricus (about 20 feet) and the last 7 feet contain 

 abundant fragments of Asaphus platycephalus. 



South of ^Mullen bay in the Westport area, Trenton beds con- 

 taining Parastrophia hemipiicata come up for a short 

 distance along the lake shore, and the shingle farther south as 

 far as the Potsdam outcrop is also nearly entirely composed of 

 Trenton limestone slabs. 



About 40 feet above Mullen bay in the railroad cut and along 

 the road other small outcrops and masses of Trenton boulders are 

 found. Some of these indicate the presence of the zone of Tri- 

 nucleus concentricus. Finally the Trenton reappears 

 again close to the fault scarp of the Precambric below the mill 

 dam on Mullen creek with an outcrop of about 40 feet of impure 

 limestones and alternating shales, the former containing the com- 

 mon brachiopods and Calymmene senaria, the latter 

 a Diplograptus of the amplexicaulis type. 



Trachyte dike. On the northern side of Cole bay, in Westport, 

 there is an east and west dike of the trach}-tic rock, to which the 

 special name of bostonite was given years ago. It cuts the Beek- 

 mantown limestone, but inasmuch as the same kind of igneous rock 

 is elsewhere found penetrating the Utica slate, it undoubtedly marks 

 a post-Ordovicic outbreak. Full details of these and associated 

 eruptives will be found in the citation below.^ The dike at Cole 

 bay is a pale, gray felsitic rock, when unweathered, but is dark 

 brown on the exposed surfaces. In thin section it presents a 

 mass of tiny feldspar crj^stals, apparently orthoclase. The struc- 

 ture is strongly trachytic ; that is, the little rods are interlaced and 

 more or less parallel, with occasional flowing arrangement. Xo 

 dark silicates remain, but a few rustv^ decomposition products sug- 

 gest their former presence but in very small amount. Xo special 

 analysis has been made, but those which have been prepared from 

 other dikes lead us to believe that the silica is in the neighborhood 

 of 60 per cent; the alumina 20; the potash and soda about 5 each; 

 while the other components make up the balance. This dike is the 

 most southerly of the bostonites thus far discovered. These pecu- 

 liar dikes are rare types of eruptives and possess much petrographic 

 interest. 



- Kemp. J. F. & Marsters, V. F. The Dikes of the Champlain Valley. 

 U. S. Geol. Sur. Bui. roj. 



