The diabase occurs only as an intrusive rock in the vicinity of the Catoctin Belt.
"History and Comprehensive Description of Loudoun County, Virginia" by James W. Head
A course gneiss is the predominant rock, but is associated with garnetiferous mica-schists and much intrusive granite.
"Encyclopaedia Britannica, 11th Edition, Volume 4, Part 3" by Various
Such intrusive rocks are porphyries, diabases, etc.
"The Economic Aspect of Geology" by C. K. Leith
The intrusive rocks predominate over the schists in extent of area covered.
"Encyclopaedia Britannica, 11th Edition, Volume 9, Slice 1" by Various
Intrusive and volcanic rocks of great variety and of various ages occur.
"Encyclopaedia Britannica, 11th Edition, Volume 5, Slice 5" by Various
We can only rely implicitly on this test where the volcanic rocks are contemporaneous, not where they are intrusive.
"A Manual of Elementary Geology" by Charles Lyell
This granite he found to be intrusive into these rocks, and into the much more ancient gneiss on which they rested.
"Charles Lyell and Modern Geology" by Thomas George Bonney
Intrusive rocks offer, as a rule, some contrasts in texture to contemporaneous masses.
"Geology" by James Geikie
The rocks of Labrador are largely metamorphic, but include ancient igneous intrusions, and are hard and resistant.
"North America" by Israel C. Russell
There are as well intrusive veins of igneous rocks.
"Cornwall" by Sabine Baring-Gould