ADIRONDACK BASIC INTRUSIVES wT 
In thin-section the plagioclase is seen to range from oligoclase 
to labradorite, and the analysis and mode show that the average 
composition is that of a basic andesine. 
The high TiO, of the analysis shows either ilmenite or very 
titaniferous magnetite, the ilmenite being far more probable 
because of the difficulty of otherwise accounting for such a low con- 
tent of Fe,0,. 
THE DIABASE 
MODE OF OCCURRENCE AND DISTRIBUTION 
In striking contrast with the neighboring gabbro, the diabase 
invariably occurs in typical dikes which have clearly broken 
through narrow fissures in the country rock. They vary in length 
from 20 or 30 feet to 200 yards, and in width from 5% inches to 
4o feet. 
The chief features of occurrence are brought out in the follow- 
ing description of the largest dike of the region which lies at the 
western base of Heath Mountain or 3 miles west-northwest of 
Warrensburg. This dike has a maximum width of 4o feet and a 
length of 200 yards. It is fine to medium grained toward the 
interior and very 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 number of small tongues, from one 
inch to three or four 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 lime- 
stone. This large dike strikes across the foliation almost at right 
angles. 
One and one-half miles southeast of Johnsburg a diabase dike, 
24 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. 
Cutting gabbro stock No. 2 (accompanying map) there is a 
typical diabase dike 4 feet wide, with fine grained borders, and 
with very sharp contacts against the gabbro. 
