MAGNETITE IRON DEPOSITS OF SOUTHEASTERN NEW YORK 37 
development in but very few areas, for they are almost invariably 
interleaved with and extensively injected by magmatic material 
similar in composition to the greater igneous bodies which consti- 
tute so large a part of the Highlands complex. 
The usual habit of these complex gneisses is a banded one with 
alternating bands of lighter and darker color corresponding to 
variations in the mineral composition. Some of the bands are com- 
posed of essentially nearly pure quartz and feldspar, others are 
largely hornblende or biotite, others mixtures of these with quartz 
and feldspar, and occasionally bands of feldspar and epidote are 
encountered. 
The gneisses of sedimentary origin may possibly include more than 
one stratigraphic group, but in many places they closely resemble 
the Fordham gneiss of southern Westchester county classed by 
Berkey °° as probably equivalent to the Grenville gneiss of the 
Adirondacks. Small lenses of interbedded crystalline limestone, in 
some instances highly silicated (sic. not silicified) accompany the 
gneiss and are to be regarded as an integral part of the same sedi- 
mentary series. There seems to be some evidence to show that these 
sediments were not only metamorphosed, but folded as well, before 
the invasion of the igneous material which now forms so large a part 
of the Highlands massif. That they exerted strong structural control 
is strikingly evident. 
They suffered lit-par-lit injection, and in many places xenolithic 
blocks, apparently detached, of the older and strongly metamorphosed 
rocks, swamped in the invading igneous masses, strike not only with 
the structural trend of the region as a whole, but each strikes in 
accordance with other, and apparently isolated, blocks. It would 
seem that a previously folded and metamorphosed series (the 
Grenville) had been invaded by a magma of exceedingly corrosive 
and insidious activity, capable of penetrating and assimilating the 
rocks through and into which it came, to an extreme degree. But 
the action was not the violent stoping postulated by Daly”; 
apparently it was instead a slow, quiet, but irresistible affected, 
attendant injection, and assimilation somewhat irregularly affected, 
so that here and there roof-pendants of the old gneisses were left, 
swamped in and surrounded by the invading mass which did not 
90 Berkey, C. P. Structural and Stratigraphic Features of the Basal 
Gneisses of the Highlands. N. Y. State Mus. Bul. 107. 10907. 
91 Daly, R. A. Igneous Rocks and Their Origin; New York, 1914, p. 194- 
ZOSMENISOP BU TOON WSN GwSstOoO2 ssp) O2=histiyAme;n OUT SGiiiv, 15) and 
vy. 16, 1903, and v. 26. 1908. 
