Preanger

On the north shore of Lake Huron there is a thick series of sediments to which the name Huronian was given by Sir Wm. Logan. These have not as yet been satisfactorily correlated with the Timiskamian or Cobalt series; unconformities have been found within them but these are of minor importance compared with that at the base of the Timiskamian or that at the base of the Cobalt series.

The Keweenawan in N.-E. Ontario is represented by the Nipissing diabase at Cobalt and by the nickel eruptive (norite micropegmatite) at Sudbury. On the shores of Lake Superior the Keweenawan consists of a great thickness of sediments and lava flows. A minor unconformity has been found to exist be tween the Keweenawan and the Animikean. Granites are found in the Sudbury area, Ontario, cutting the Keweenawan nickel eruptive, and they belong to the third period of granitic intru sion, named the Killarnean. The pre-Cambrian era in North Eastern Ontario was closed by the intrusion of fresh olivine dia base dikes which intersect all the older rocks and which in certain regions may be traced for many miles.

In that almost unexplored region of the Canadian Shield be tween the Atlantic on the E. and Great Slave and Great Bear lakes on the W. there is very little known regarding the rocks. They appear to consist largely of granites and gneisses ; and a few isolated areas may correspond to the Keewatin and Timis kamian series. Late pre-Cambrian sediments relatively undis turbed occur along the Labrador coast, inland to the South of Ungava Bay, on the E. coast of Hudson Bay, on the Belcher Islands of that coast, on the S. side of Athabaska Lake, and in the Coppermine River area—where the rocks attain the thickness of over 9 miles and are known as the Coppermine River series. They resemble the Keweenawan series on the S. shore of Lake Superior, and like that series the Coppermine River group also contains deposits of native copper.

In the western part of Canada, including British Columbia and the Yukon, there are areas of pre-Cambrian rocks. In southern British Columbia the pre-Cambrian sediments attain great thick ness, the Purcell series consisting of more than 20,000 ft. of sediments, mostly argillaceous quartzites.

United States.

In the Lake Superior region, where the pre Cambrian rocks have been intensively studied by Leith and Van Hise, they have been sub-divided as follows, commencing with the youngest : There have been, as in Ontario, three periods of granitic intru sions, namely, at the close of the Archaean, at the close of the Lower—Middle Huronian, and in Keweenawan times. The first

has been called the Laurentian. The other two are not specifically named, but in general correspond respectively to what has been called Algoman and Killarnean in Ontario. The Huronian is divided into three unconformable series, lower, middle and upper. The lower appears to correspond, in certain areas, with the Timiskamian of Ontario. Two major unconformities are recog nized, one at the base of the upper, and one at the base of the lower (Knife Lake Series). Two others, at the base of the Keweenawan and at the base of the middle Huronian, are distinct but not conspicuous. The two major unconformities appear to correspond with those in Ontario at the base of the Animikean and of the Timiskamian respectively.

In Montana and northern Idaho there is a very thick series, consisting largely of sandstones and shales, known as the Beltian series; the pre-Cambrian sediments in the Grand Canyon of Arizona—the Grand Canyon series—are magnificently exposed; in the Adirondacks (New York State) the Grenville sediments are found ; and in Pennsylvania, Maryland and Northern Virginia the Glenarm series of metamorphosed pre-Cambrian sediments.

British Isles.

In the British Isles the pre-Cambrian rocks have their greatest development in Scotland, but isolated areas of considerable interest and importance are also known in the Isle of Anglesey, N. Wales, S. Wales, the Welsh Borderland and the English Midlands.

Taken as a whole they fall into two main groups, an older group composed mainly of highly crystalline Schists and Gneisses, separated by a great unconformity from a younger relatively unaltered group of sediments and volcanic rocks. The older group of Schists and Gneisses has its most extensive development in the Highlands of Scotland of which it forms by far the larger part ; here it includes the Glen Elg Schists, the Lewisian Gneisses, a highly altered plutonic complex, the Moinian, and the Dalradian Schists. The Glen Elg Schists, like the Moinian and Dalradian, consist in the main of a series of highly metamorphosed sedi ments of various kinds, though the Dalradian also contains schists of volcanic and hypolysed origin. The relative ages of these Schist Groups has afforded a matter for considerable contro versy, and the matter cannot yet be regarded as definitely settled.