Bigsby— On the Laurentian Formation. To 
in prolonged hill-ranges, twelve miles broad; it occurs at 
Chateau Richer, below Quebec, and from thence extends at intervals 
NE. into the Straits of Belleisle, and so northwardly along the 
eastern shores of Labrador, according to the investigations of Pro- 
fessor Hind, of Toronto, and as shewn by the specimens of labra- 
dorite, hypersthene, &c., brought from thence by Admiral Bayfield, 
and now in the Museum of the Geological Society of London. In 
the Georgian Bay of Lake Huron I found it in exclusive possession 
of a group of islets, five miles long. A careful analysis of anortho- 
site by Sterry Hunt shows* that it is a felspar with a lime-base 
and a small per-centage of potash orsoda. He finds from 7 to 15 per 
cent. of lime in most of these rocks. Being of variable composition, 
they have received several names—andesine, labradorite, hyper- 
sthene, vosgite, &c.; the difference being chiefly in the proportions 
of the ingredients. The anorthosite, as found in rock-masses 
north of Montreal, is composed chiefly of felspar, with a little 
black mica, green pyroxene, some epidote, garnet, ilmenite 
(titanitie iron), iron-pyrites, and calespar as accidental minerals. 
Its texture is not always the same: sometimes the mass is a 
confusedly crystalline aggregate, exhibiting cleavage-surfaces three 
or four lines in diameter; with a fine-grained paste, somewhat 
calcareous, in the interstices. Occasionally the whole rock is uni- 
formly granular, with cleavable masses of felspar at intervals ; and 
its colour may be greyish and bluish white, lavender- and violet- blue 
(opalescent), flesh-red, greenish, and brownish. The crystallization 
is triclinic, which, taken with the density 2°66—2-73, shews this 
rock to belong to the same group as albite and anorthite,—in fact, a 
mixture of the two (S. Hunt). The anorthosite has been seen 
to pass into orthoclase-gneiss (that is, gneiss containing potash- 
felspar). There is no doubt of the presence of these lime-felspars in 
Norway and Scotland ; and the rose-coloured crystals of the red an- 
tique porphyry of Egypt, according to the analysis of M. Delesse, 
have the same composition (S. Hunt). 
The other associated rocks of the Laurentian Gneiss of Canada, 
possessing no features of novelty, will be passed over here in silence; 
but still it seems necessary even in this very brief description to 
state that on the Lower Ottawa syenite forms a large boss, with a 
porphyritic core, in this gneiss; and that there are dykes and veins 
belonging to four epochs in that district,—of dolerite (going east 
and west), and of syenite, cutting the doleritic dykes; felsite-por- 
phyry, cutting the syenite; and augitic trap, traversing the Lau- 
rentian in all directions ;} the last, it is important to remark, in- 
vading the Lower Silurian rocks. Iron-ores, metallic sulphurets, 
graphite, fluates, and phosphates occur abundantly in the Laurentian 
here and in other countries. 
Distribution. —Laurentian rocks occur in many widely scattered 
localities in the United States, as in Pennsylvania,t{ Missouri, the 
* Geol. Report, 1854, pp. 374, 383. t Report, 1863, p 37. 
~ Compare Rogers’s Final Rep. Geol. Pennsylvania, vol. i. p. 62; ii. pp. 744, 
747, &e. 
