Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/361

 the thick-bedded limestone are found in the Laurentian area of Madoc, at a considerable distance from the main mass. In a lecture delivered at Madoc in 1866, 1 called attention to these outlying patches. One of them occurs about a mile south of the Richardson gold-mine, and the other at a distance of about 2 miles S.E. of the former mass. The N. and N.E. sides of these outliers present prominent escarpments, whilst the other sides have a gentle slope. Other smaller outliers of limestone are scattered over the Laurentian area ; and all are of interest as attesting the extensive denudation which the country has suffered.

At the base of the Trenton group there is found in certain localities of the North Riding a thin band of grey limestone, having so extremely fine a texture as to render it well adapted for use as a lithographic stone.

In the township of Hungerford the Trenton limestone is occasionally underlain by an unfossiliferous calcareous sandstone, supposed to represent the Calciferous Sand-rock and Potsdam Sandstone, which form the base of the Lower Silurian formation.

4. Laurentian. — The Lower Silurian beds usually rest unconformably upon a very irregular surface formed by the denuded edges of a large group of highly inclined strata of metamorphic rocks, which have been referred provisionally to the Lower Laurentian formation. These rocks are exposed over a large portion of the North Riding, and consist of a very diversified series of micaceous, hornblendic, and chloritic schists, interstratified with beds of granular and crystalline limestone, and penetrated by bosses of syenitic and gneissoid rocks. Bands of conglomerate occur locally, and consist of quartzose, felspathic, and calcareous pebbles, imbedded in a matrix of micaceous schist or of dolomitic limestone. Most of the stratified Laurentian rocks exhibit evidence of having been highly disturbed, the dip being extremely irregular, and often at a very high angle. An apparent inversion of the rocks may be seen in the adjoining townships of Tudor and Madoc. Traces of an organic structure referred to Eozoon Canadense have been detected by the Geological Survey of Canada in the limestones of Madoc and Tudor; but it is supposed that these rocks may be placed on a higher horizon than the Eozoonal limestones of Grenville. Indeed Sir William Logan admits that the stratigraphical position of the crystalline rocks of Hastings is by no means satisfactorily determined ; but he adds that " it would be premature to remove them from the horizon in which they have been provisionally placed."

In addition to the extensive development of these so-called Laurentian rocks in the northern townships, domes of similar syenitic and gneissoid rocks are exposed in several parts of the Trenton- Limestone area to the south, where the overlying limestone has been planed down or removed by denudation.

It has been suggested that certain labradorite rocks forming a range in the township of Tudor, known locally as the " Hole in the Wall," may be regarded as outlying masses of the Labrador or Upper Laurentian series.