ADIRONDACK BASIC INTRUSIVES 173 
excellent diabasic texture visible even to the naked eye. Judging 
by the extinction angles, the broad laths of somewhat decomposed 
plagioclase range from andesine to labradorite in composition. 
Pale reddish-brown augite, in stout prisms, shows a very faint 
pleochroism. It exhibits good cleavage and sometimes good crystal 
boundaries. The biotite is much changed to chlorite and stained 
with black iron oxide. The magnetite often shows transition to 
leucoxene. Apatite occurs in tiny needles, and pyrite and quartz 
in small irregular grains, the latter probably being of secondary 
origin. 
TABLE III 
MINERALOGICAL COMPOSITION OF THE DIABASE 
Sites | Seo eee ee Glassy 
No. | Labra- | Ausite | Biotite fa Pyrite |Ground-| Quartz | Apatite 
dorite HEISE 
lio 60 he ae eee 8 47 25 DDG 4 3 little | little 
37 
206 foals caleaeiinchiontS No, IP little |... 2 | little 
4o 
SS . . 
25 oie eee 7 55 | mostly chlorite 5 ae a. little | little 
many 
El, 9.0 Ov EERO eR Io 5 5 S32 specks a 85 
3 0 6:6: coir e ree fe) 55 14 or: See littles |, 25 I 
CHEMICAL COMPOSITION, NORM, AND MODE 
The diabase from the dike at the western base of Heath Moun- 
tain has been chemically analyzed for the writer by Professor 
E. W. Morley. The following table (p. 174) shows the chemical 
composition, norm, and mode, the last having been determined 
jrom thin-sections. 
Thus, according to the old qualitative classification, the rock 
is a biotite-diabase, while under the new quantitative chemical 
classification it is a biotite-camptonose. 
The amounts of SiO., Al,O;, and CaO in the analysis strongly 
bear out the determination of the plagioclase as ranging from 
andesine to labradorite. 
Such a high content of FeO in the analysis makes it certain 
that the biotite is rich in ferrous iron, since there is not enough 
ilmenite and augite to account for so much FeO. 
