Bigshy—On the Laurentian Formation. Lop 
GJ SOY 
Huronian and Cambrian rocks*) it underlies discordantly beds more 
or less fossiliferous, named ‘ Primordial’ in Bohemia, ‘ Lingula-flags ’ 
in Wales, ‘ Potsdam Sandstone’ in North America. Such is its posi- 
tion in the crust of the earth. To Sir William Edmond Logan be- 
longs the honour of having distinctly announced the peculiar signifi- 
cance of the ‘ Laurentian :’ geologists not having recognized the fact, 
that it consists of altered sediments, once possibly rich with organ- 
isms, and still leaving us far from the beginning of life. For 
the purpose of study it will be well to consider the Canadas as pos- 
sessing the type of this formation ; for it there occupies a space of 
200,00) square miles, in ample development (40,000 feet thick) ; 
and it has there been subjected for many years to the systematic 
examination of observers of the highest qualifications; mainly at 
the expense of the Canadian people. 
Sir W. E. Logan says of these Laurentian or oldest sedimentary 
rocks: ‘ They are altered to a highly crystalline condition, and are 
composed of felspathic rocks, interstratified with important masses 
of limestone and quartzite. Great vertical thicknesses of the series are 
composed of gneiss containing chiefly orthoclase or potash-felspar ; 
while other great portions are destitute of quartz and composed chiefly 
of a lime-soda-felspar, varying in composition from andesine to anor- 
thite,t and associated with pyroxene or hypersthene. This rock,’ he 
adds, ‘we shall distinguish by the name of anorthosite.’ The chief 
divisions then, without reference to any order of superposition, are— 
1. Orthoclase-gneiss, sometimes granitoid; with quartzite, horn- 
blendic and micaceous schists, pyroxene, and garnet-rock. 
2. White crystalline limestone, and dolomites, in numerous thick 
beds, containing serpentine, pyroxene, hornblende, mica, graphite, 
iron-ores, apatite, fluor, &c., and interstratified with bands of gneiss. 
3. Lime-felspar-rock, or anorthosite, containing hypersthene, ilmen- 
ite, pyroxene, hornblende, graphite, &c. These thr ee groups are 
traversed by granitic and metalliferous veins. 
1. Gnetss.§—The Laurentian Gneiss of Canada is grey or light- 
red, and is composed of quartz and felspar, in minute combination, 
and many thin layers of black mica (often mixed with or replaced by 
little plates of hornblende) running along the foliation. ‘This rock 
is greatly contorted, and for miles together, as in north-east Lake 
Huron, is ribboned with pretty festoonings: the lamine are re- 
garded as sedimentary layers. ‘The strike is mostly NE.-SW., or 
NNE.-SSW. For practical purposes, Logan divides this gneiss 
into two principal kinds—(1) the granitic or micaceous, and (2) 
* See Geol. Report Canada, 1863, p. 50; and Quart. Journ. Geol. Soc., vol. xix. 
. 36. 
: t See the successive Reports on the Geology of Canada, by Sir W. E. Logan and 
his colleagues, especially the large comprehensive Report for 1863, as well as the 
Descriptive Catalogue of Canadian Minerals and Rock, Internat. Exhib. 1862. 
t Report, 1863, p. 22. These varying triclinic felspars being anorthic in erystal- 
ization and approaching anorthite in composition, Delesse proposed to give them the 
name ‘anorthose,’ and ‘anorthosite’ to the rock-masses formed of them. 
§ By ‘ gneiss’ is meant a rock consisting of the same materials as granite (with 
or without hornblende), but with a lamellar structure. 
