On a nearer acquaintance, the Laurentian country appears as a broken and hilly upland and highland district, clad in its pristine state with magnificent forests, but affording few attractions to the agriculturist, except in the valleys, which follow the lines of its softer beds, while it is a favourite region for the angler, the hunter, and the lumberman. Many of the Laurentian townships of Canada are, however, already extensively settled, and the traveller may pass through a succession of more or less cultivated valleys, bounded by rocks or wooded hills and crags, and diversified by running streams and romantic lakes and ponds, constituting a country always picturesque and often beautiful, and rearing a strong and hardy population. To the geologist it presents in the main immensely thick beds of gneiss, and similar metamorphic and crystalline rocks, contorted in the most remarkable manner, so that if they could be flattened out they would serve as a skin much too large for mother earth in her present state, so much has she shrunk and wrinkled since those youthful days when the Laurentian rocks were her outer covering. ([Fig. 3.])
The elaborate sections of Sir William Logan show that these old rocks are divisible into two series, the Lower and Upper Laurentian; the latter being the newer of the two, and perhaps separated from the former by a long interval of time; but this Upper Laurentian being probably itself older than the Huronian series, and this again older than all the other stratified rocks. The Lower Laurentian, which attains to a thickness of more than 20,000 feet, consists of stratified granitic rocks or gneisses, of indurated sandstone or quartzite, of mica and hornblende schist, and of crystalline limestones or marbles, and iron ores, the whole interstratified with each other. The Upper Laurentian, which is 10,000 feet thick at least, consists in part of similar rocks, but associated with great beds of triclinic feldspar, especially of that peculiar variety known as labradorite, or Labrador feldspar, and which sometimes by its wonderful iridescent play of colours becomes a beautiful ornamental stone.
I cannot describe such rocks, but their names will tell something to those who have any knowledge of the older crystalline materials of the earth’s crust. To those who have not, I would advise a visit to some cliff on the lower St. Lawrence, or the Hebridean coasts, or the shore of Norway, where the old hard crystalline and gnarled beds present their sharp edges to the ever raging sea, and show their endless alternations of various kinds and colours of strata often diversified with veins and nests of crystalline minerals. He who has seen and studied such a section of Laurentian rock cannot forget it.
Fig. 3. Section from Petite Nation Seigniory to St. Jerome (60 miles). After Sir W. E. Logan.
(a, b.) Upper Laurentian. (c.) Fourth gneiss. (d′.) Third limestone. (d.) Third gneiss. (e′.) Second limestone. (x.) Porphyry. (y.) Granite.
All the constituents of the Laurentian series are in that state known to geologists as metamorphic. They were once sandstones, clays, and limestones, such as the sea now deposits, or such as form the common plebeian rocks of everyday plains and hills and coast sections. Being extremely old, however, they have been buried deep in the bowels of the earth under the newer deposits, and hardened by the action of pressure and of heat and heated water. Whether this heat was part of that originally belonging to the earth when a molten mass, and still existing in its interior after aqueous rocks had begun to form on its surface, or whether it is a mere mechanical effect of the intense compression which these rocks have suffered, may be a disputed question; but the observations of Sorby and of Hunt (the former in connection with the microscopic structure of rocks, and the latter in connection with the chemical conditions of change) show that no very excessive amount of heat would be required. These observations and those of Daubrée indicate that crystallization like that of the Laurentian rocks might take place at a temperature of not over 370° of the centigrade thermometer.
The study of those partial alterations which take place in the vicinity of volcanic and older aqueous masses of rock confirms these conclusions, so that we may be said to know the precise conditions under which sediments may be hardened into crystalline rocks, while the bedded character and the alternations of different layers in the Laurentian rocks, as well as the indications of contemporary marine life which they contain, show that they actually are such altered sediments. (See [Note D].)