CURRENT PRE-CAMBRIAN LITERATURE 851 
clase gneisses, presenting great variations both in structure and com- 
position, with which are associated crystalline limestones, quartzites, 
and amphibolites. In certain parts of the area two divisions can be 
recognized in the Laurentian: an upper series, characterized by the 
presence of crystalline limestones, quartzites, and gneisses of sedi- 
mentary origin with a banded structure, called the Grenville series ; 
and a lower series of gneisses in which no limestone, etc., occur, and 
which possess a foliated rather than a banded structure, known as the 
Fundamental Gneiss. Grenville rocks are recognized south of Raw- 
don and in the westerly portion of the St. Sauveur district. The 
Fundamental Gneiss apparently occupies much of the St. Jerome dis- 
trict. However, it has been found impossible to separate the two 
series and delimit them on the map. 
The composition of most, if not all, of the gneisses belonging to 
the Fundamental Gneiss can be paralleled among the igneous rocks, 
and it is concluded that many of these gneisses, at least, were of igne- 
ous, probably of intrusive, origin. In the Grenville also some of the 
gneisses are of igneous origin. However, many are believed to 
be of sedimentary origin, for the following reasons: (1) they are 
associated with numerous and heavy beds of limestones and quartzite ; 
(2) they have a prevailing banded character, accompanied by a very 
extensive recrystallization ; (3) graphite is of frequent occurrence in 
them; (4) chemical analyses show that they have the composition, not 
of igneous rocks, but of sedimentary sands and muds. 
The quartzite is sometimes pure, but frequently holds garnet, silli- 
manite, or other minerals. The limestones are coarsely crystalline 
marbles, sometimes pure, but at other times including grains of 
quartz, pyroxene, phlogopite, graphite, and other minerals. 
The anorthosite belongs to the gabbros, but is characterized by the 
great preponderance of plagioclase feldspar, which is often so abundant 
as to make up the entire rock. At its contact with the gneisses are 
many contact phases. The anorthosite has been squeezed and foliated, 
together with the gneisses which it cuts, and it is concluded that its 
intrusion antedated at least the termination of the great earth move- 
ments which affected the Laurentian in pre-Potsdam times. In pro- 
portion as the anorthosites exhibit granulation they become light 
colored, some of the most metamorphosed ones resembling marble in 
appearance, although chemically they do not differ from the less 
modified anorthosites. 
