GEOLOGY OF THE NORTH CREEK QUADRANGLE 39 



fine grained along the borders. It breaks through both Grenville 

 and granite gneisses and the contacts are everywhere perfectly sharp, 

 there being no evidence whatever of contact metamorphism. A num- 

 ber of small tongues, from i inch to 3 or 4 feet wide, branch off 

 the large dike and extend as much as 25 or 30 feet into the country 

 rock. One of these branches cuts a pegmatite dike and another 

 cuts Grenville limestone. This large dike strikes across the folia- 

 tion almost at right angles. 



One and one-half miles southeast of Johnsburg a diabase dike, 

 23^ feet wide and 60 feet long, cuts Grenville quartzite parallel to 

 the foliation. All of this rock is fine grained but exceptionally so 

 at the contacts, and on one side an inch wide zone of basaltic glass 

 or obsidian is perfectly developed with some very small tongues of 

 glass extending into the country rock. 



A typical diabase dike 4 feet wide cuts the gabbro stock three- 

 quarters of a mile south of Warner pond. The dike has fine grained 

 borders, sharp contacts against the gabbro, and is clearly traceable 

 for 150 feet or more. 



In all eleven diabase dikes were found, being well scattered over 

 the quadrangle. Most of them cut across the foliation of the coun- 

 try rock at high angles, thus differing from the gabbros, and they 

 probably have been forced up along joint planes. In nine of the 

 eleven occurrences the dikes strike northeast and southwest which 

 is quite the rule for such dikes in the eastern Adirondacks. So far 

 as can be determined, these dikes all come up vertically through 

 the country rock. 



Megascopic and microscopic features 



The diabase is a very dark bluish gray to almost black rock which, 

 in all exposures, is hard and fresh except for the immediate surface 

 which is often weathered to reddish brown. 



The granularity and texture vary from glassy to very fine grained 

 to medium grained diabasic, the finer grained rock being wholly 

 confined to the borders and the diabasic texture nearly always being 

 just visible to the naked eye in the typical medium grained rock. 

 Except for the above named differences the diabase shows no facies 

 whatever visible to the naked eye, and this again is in marked con- 

 trast with the gabbros. The diabase is wholly devoid of any meta- 

 morphism and inclusions of country rock are never found. The 

 only minerals recognizable by the naked eye arc the tiny feldspar 

 laths and an occasional pyrite speck. 



