There is, perhaps, no part of the world, certainly none familiar to science, where the early geological periods can be studied with so much ease and precision as in the United States.  Along their northern borders, between Canada and the United States, there runs the low line of hills known as the Laurentian Hills.  Insignificant in height, nowhere rising more than fifteen hundred or two thousand feet above the level of the sea, these are nevertheless the first mountains that broke the uniform level of the earth’s surface and lifted themselves above the waters.  Their low stature, as compared with that of other more lofty mountain-ranges, is in accordance with an invariable rule, by which the relative age of mountains may be estimated.  The oldest mountains are the lowest, while the younger and more recent ones tower above their elders, and are usually more torn and dislocated also.  This is easily understood, when we remember that all mountains and mountain-chains are the result of upheavals, and that the violence of the outbreak must have been in proportion to the strength of the resistance.  When the crust of the earth was so thin that the heated masses within easily broke through it, they were not thrown to so great a height, and formed comparatively low elevations, such as the Canadian hills or the mountains of Bretagne and Wales.  But in later times, when young, vigorous giants, such as the Alps, the Himalayas, or, later still, the Rocky Mountains, forced their way out from their fiery prison-house, the crust of the earth was much thicker, and fearful indeed must have been the convulsions which attended their exit.

[Illustration:  A PHYSICAL MAP OF THE UNITED STATES.]

The Laurentian Hills form, then, a granite range, stretching from Eastern Canada to the Upper Mississippi, and immediately along its base are gathered the Azoic deposits, the first stratified beds, in which the absence of life need not surprise us, since they were formed beneath a heated ocean.  As well might we expect to find the remains of fish or shells or crabs at the bottom of geysers or of boiling springs, as on those early shores bathed by an ocean of which the heat must have been so intense.  Although, from the condition in which we find it, this first granite range has evidently never been disturbed by any violent convulsion since its first upheaval, yet there has been a gradual rising of that part of the continent; for the Azoic beds do not lie horizontally along the base of the Laurentian Hills in the position in which they must originally have been deposited, but are lifted and rest against their slopes.  They have been more or less dislocated in this process, and are greatly metamorphized by the intense heat to which they must have been exposed.  Indeed, all the oldest stratified rocks have been baked by the prolonged action of heat.