THE ORIGIN OF THE INCLUSIONS IN DIKES 



SIDNEY POWERS 



Geological Museum, Cambridge, Massachusetts 



PART II 



(C) Many inclusions have risen. — 



Shelburne Point, Vermont: On Shelburne Point and on Nash's 

 Point, near-by, a few miles south of Burlington, Vermont, are several 

 inclusion-bearing dikes of great interest. They have been described 

 by Hitchcock' and by Kemp and Marsters.^ On both sides of 

 Shelburne Point are outcrops of a 20-foot bostonite dike filled with 

 angular inclusions of Middle Ordovician shale and red Cambrian 

 quartzite. These inclusions have sharp edges and do not appear 

 to have been noticeably altered by the bostonite. On Nash's 

 Point is another bostonite dike, about 12 feet wide and vertical, 

 with chilled margins 2 feet and i foot wide on the respective sides 

 of the dike, and a mass of fragments in the middle. A part of the 

 fragments are quite angular and a part decidedly rounded. They 

 vary in size from a fraction of an inch to 4 or 5 inches in length, but 

 Kemp and Marsters found one piece of norite 18 feet in diameter. 

 They consist of garnetiferous hornblende schist, pre- Cambrian 

 norite, quartz, grey sandstone, red Cambrian sandstone, Trenton 

 shale, and black Kmestone, cemented by a bostonite groundmass. 

 Kemp and Marsters add: "Under the microscope, sections of 

 norite show plagioclase and garnet, all exhibiting the results of 

 dynamic action. Sections of the red quartzite have the usual 

 fragmental character, with the evidences of strain less developed." 

 The presence of the norite and schist inclusions show that the dikes 

 have come through an indefinite amount of the pre-Cambrian as 

 well as through Cambrian sandstone and Ordovician limestone into 



' "On Certain Conglomerated and Brecciated Trachytic Dikes in Vermont," 

 Proc. Amer. Ass. Adv. Sci., XIV (i860), 156. 



= "Trap Dikes of the Lake Champlain Region," U.S. Geol. Siirv., Bull. lo-j. 



166 



