OF PRE-CAMBRIAN ROCKS 207 
Rusty micaceous rocks with a peculiar yellow color on exposed 
surfaces are also frequent, although not in great thickness. They 
and other schistose, micaceous gneisses are believed to be recrystallized 
sediments. 
Beneath the above, and sometimes interstratified with them, are 
more massive gneisses of a general granitic composition. While 
quartz and feldspar are the principal minerals, dark silicates also 
appear of the following kinds: emerald-green augite, brown horn- 
blende, and biotite. Several varieties of gneiss are known depending 
on the relative proportions of these minerals. The iron ores are 
associated with them in almost all cases. It is a doubtful point as to 
whether these gneisses are in any cases sediments. They may be 
mashed, intrusive, granitic rocks in which the foliation is secondary, 
or they may be extreme phases in the metamorphism of arkoses or 
acidic volcanic tufts. 
In the Adirondack Mountains the series is invaded by enormous 
bodies of intrusive rock (anorthosite, gabbro, syenite, etc.), so that 
the sedimentary portion of the series forms a relatively smaller propor- 
tion of the whole complex than in the other areas covered by the work 
of the committee. 
The undoubted sediments and their associated gneisses of uncer- 
tain origin are penetrated by several varieties of plutonic eruptives. 
The oldest constitute the anorthosite series, and embrace rocks from 
aggregates of nearly pure plagioclase to fairly rich mixtures of augite, 
hypersthene, and ilmenite with the plagioclase. The borders of 
the great bathyliths tend to be more basic than the centers. 
The next in time among the intrusives constitute the syenite 
series. The typical rocks consist of microperthite, augite, horn- 
blende, biotite, and varying amounts of quartz. Subordinate varieties 
are as rich in quartz as typical granites. Near the contacts with 
anorthosites and limestones garnets become numerous. 
The third eruptive series consists of typical granites of both horn- 
blendic and micaceous varieties. Areally the granites are of small 
extent, but they appear in scattered exposures outside the anorthosites. 
The fourth series consists of dark basic gabbros, which are cer- 
tainly later than the others, except perhaps the granites. No inter- 
sections with the latter are known. The basic gabbros appear in 
