1895. ] NEW YORK ACADEMY OF SCIENCES. 201 
DIABASE. 
Except the felsites this is the most abundant rock of the Cold- 
brook group, and forms also a considerable part of the rocks re- 
ferred to the Coastal series in 1887-8. It occurs as surface flows 
as well as intruded sills and dykes. At Racehorse Point, east 
of St. John City, it crops out on the shore as a heavy dy ke in 
the coarse green and red sandy slates that are overturned on the 
fine grey slates of the St. John group.* This dyke has baked 
the adjoining slate into a hornstone very like the edges of the 
igneous rock itself, and the contact of the two is not easy to dis- 
tinguish. Dr. Gesner in 1840+ mentions it as a dyke, but sup- 
poses that it has fused the slate near by, and thus effaced the 
line of contact. The Survey Reports} do not recognize it asa 
dyke, but consider it, apparently, as an altered sediment or ashi- 
rock. In thin section, however, it is unmistakably a diabase, and 
on careful examination the line of contact with the slates was 
traced on each side. Under the microscope this contact is per- 
fectly sharp and clear, the igneous rock being dense, almost 
opaque, with scattered plagioclase phenocrysts. The slate does 
not show much change; the patallel arrangement of the granules 
is not noticable, and minute grains of a brightly polarizing min- 
eral are developed near the edge. The diabase of the central 
parts of the dyke is tolerably fresh; the augite occasionally 
shows crystal outlines, but mostly moulds the feldspar. No 
olivine was observed, but a peculiar oil-green substance, indis- 
tinctly fibrous or matted, with a high refractive index and bright 
aggregate polarization, appears occasionally , and is probably an 
alteration product of olivine, perhaps allied to Becke’s pilite. 
At some little distance back from the shore, near the peniten- 
tiary, the diabase crops out again, here being in part a surface 
flow, finer grained and strongly vesicular, and separated by a 
sharp line of contact from the coarser, non-vesicular diabase 
lying northwest of it. The latter contains included masses of 
white-weathering flinty felsite, which seem to belong just north 
of it, and are very like the supposed ash-rocks underlying the 
Etcheminian north of the city. 
From the foregoing it is clear that a part of the diabase, the 
dyke on the shore and the heavier dyke or sill back of it, must 
be later than the slate and felsite through which they cut. Of 
the relative age of the vesicular diabase there is no good evi- 
dence ; though it appears to be older than the sill. The slate is 
* Trans. Roy. Soc. Can. 1890, Sec. 4, p. 127. 
+ Geol. Sur. Prov. New Bruns., Rep. 1840, p. 11. 
t Geol. Sur. Canada, Rep. 1871, p. 137. 
