112 NEW YORK STATE MUSEUM 



Dikes. A minor feature of the geology of the region is the occur- 

 rence of dikes which are specially common in the vicinity of the 

 ore bodies. They belong to two series of intrusions, an older 

 represented by granite and a younger consisting of diabase. 



The granite dikes vary from a few inches to several feet in width. 

 In appearance they resemble the reddish gneiss, from which they 

 can be distinguished, however, quite readily by their finer and more 

 massive texture. Mineralogically they consist of quartz and feld- 

 spar, with subordinate augite, magnetite, titanite and zircon. The 

 feldspar is prevailingly orthoclase, but a triclinic variety, probably 

 oligoclase, is usually present. The dikes are almost identical in 

 composition with the Birch hill granite which strongly points to 

 the conclusion that the two rocks are genetically related. The 

 granite dikes can be best observed at the Williams and Burden 

 openings. At the former locality there are several running paral- 

 lel to or slightly diverging from the course of the ore body. The 

 only contact effect consequent upon the intrusion of the dikes is 

 a slightly bluish tint assumed by the magnetite. 



The diabase dikes occur in numbers both on the surface and in 

 the underground workings. They range up to 15 feet thick, the 

 largest one observed being near slope 15. They do not follow any 

 one direction, though the majority of them have a nearly east-west 

 strike, while most of the others run about n. 30 w. New dikes 

 are frequently encountered in the course of the mining operations. 



The petrography of the dikes has been described by Kemp and 

 Marsters, 1 who state that they are all diabase, though showing 

 some variation in individual cases. One dike from the Hall slope 

 is said to be characterized by the presence of small hornblende crys- 

 tals in addition to those of augite, showing a transition to camp- 

 tonite. The writer's observations are in accord with the view cited 

 as to the diabase nature of the dikes. With one or two exceptions 

 examination of thin sections revealed little that is noteworthy in 

 their composition or texture. A small dike from slope 4 is charac- 

 terized by a pronounced porphyritic habit due to the inclusion of 

 augite phenocrysts in a fine ground mass of augite and plagioclase. 

 Some dikes are peculiarly rich in magnetite which has probably 

 been absorbed from the ore bodies during the period of intrusion. 

 This mineral frequently takes the unusual form of long needles 

 which are arranged in parallel groups crossing one another at 



1 The Trap Dikes of the Lake Champlain District. U. S. Geol. Sur. Bui. 

 107. 1893. p. 447-48- 



