Page:The Journal of geology (IA journalofgeology21894univers).pdf/127
- The Fundamental Gneiss may be considered as a great mass of eruptive rock, which has eaten upward and penetrated the Grenville series, while the Grenville series itself represents a series of altered sediments of Laurentian, Huronian, or subsequent age. The world wide distribution of the Fundamental Gneiss (forming as it does, wherever the base of the geological column is exposed to view, the foundation upon which all subsequent rocks are seen to rest) is opposed to this view, as is also its persistent gneissic or banded character.
The anorthosite series is a gabbro, often regularly laminated and much altered, which is intrusive within the Fundamental Gneiss and the Grenville series.
The Hastings series has a very local development. It consists largely of calc-schists, mica-schists, dolomites, slates, and conglomerates, thus containing much material of undoubtedly clastic origin. The whole district has been subjected to great dynamic action, some of the pebbles of the conglomerates being distorted in the most remarkable manner. This series may be equivalent to a part of the original Laurentian, may follow above the Grenville series, or may prove to be an outlying area of Huronian rocks folded in with the Laurentian.
The whole of the above series was cut by various acid and basic rocks, metamorphosed and folded before upper Cambrian time, since the Cambrian sediments rest upon them unconformably, and contain fragments of the lower series which show that when deposited they were in their present condition.
The roche moutonnée surface possessed by the eroded Laurentian rocks was impressed upon them in the first instance in pre-Cambrian times, for along the edge of the nucleus from Lake Superior to the Saguenay, the Paleozoic strata may be seen to overlie such surfaces showing no traces of decay, and similar to that exposed over the uncovered part of the area. To what extent the Cambrian, Devonian, and Silurian seas passed over the Laurentian cannot be determined, but it seems probable that in Cambrian times, a not inconsiderable part of the Archean Nucleus was under water, as shown by various outliers of these rocks. What evidence there is indicates that the area in later Paleozoic, Mesozoic, and earlier Tertiary times, was out of water, being subjected to deep-seated decay and denudation, culminating in the glaciation of Pleistocene times. These processes removed all but remnants of the Paleozoic strata.
Comments.—The question may perhaps be asked whether the visible contacts of the pre-Cambrian and Cambrian are sufficiently extensive to warrant the statement that the pre-Cambrian topography was similar to the present topography. May not the tendency to carry in imagination the present forms under the Cambrian have been given undue weight?