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Page:Popular Science Monthly Volume 19.djvu/246

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THE POPULAR SCIENCE MONTHLY.

once water-drifted beds metamorphosed, transfigured, as it were, by heat and pressure into this adamantine pavement; and, further, we find that they must have been so formed in the attrition and decay of yet older continents. The dim perspective opens backward to the very verge of chaos.

After deposition, and in a somewhat consolidated state, they were slowly raised, their emergence above the water accompanied a contraction of the earth's crust, and the flexible series, from top to bottom, folded up in deep and multiplied plications. Mountain-chains arose, their strata tilted up, contorted and complicated in related groups of synclinal and anticlinal axes, and, by the effective agency of heat and aqueous distillation through the myriad pores of the rock, a mineralogical change ensued. The argillaceous muds were hardened into slates and schists, the calcareous shoals became crystalline limestones, marbles, and dolomites, the siliceous bands became quartzites and sandstones, the iron slime crystallized into colossal sheets of iron-ore, magnesian sediments became serpentine, and through all there developed beautiful minerals under various associations and marking different horizons in this complex pile of natural masonry. Feldspars, pyroxene, mica, apatite, chondrodite, epidote, and garnet are a few of many which, in crevice and seam, and scattered through the matrix rock, remain as token, and possibly revelation, of the changes here enacted. From this archæan country come the magnetic oxide of the Adirondacks, the hematite of Marquette, the soft lead of Ticonderoga, the dolomite of Westchester County, the mica of North Carolina, the syenites and granites of Maine, the marbles of Vermont, the tinstone of New Hampshire, and the phosphates of Canada. Over thirty thousand feet in vertical thickness is the estimated depth of this gigantic mass—fitting foundation for the arches of the world.

Recent study, notably that of Dr. T. Sterry Hunt, separates this wonderful epoch into four secondary ones of unequal duration and varying character. First, the Laurentian, a name given by the Geological Survey of Canada and applied originally to the rocks of the Laurentian highlands, those abraded swells of land which overlook the St. Lawrence and rise in rugged grandeur four thousand feet high above the shadowed waters of the Saguenay. This primitive tract of archæan territory embraces the Adirondacks of New York, the region about Ottawa, portions of Newfoundland, and probably includes the rock assigned to this age in Massachusetts, Connecticut, and Rhode Island, and the long back which makes up the Highlands of the Hudson, the South Mountain of New Jersey, and the ridges about Richmond and Mount Roan in North Carolina. The rock is "a strong, massive gneiss, reddish or grayish in color."

Following this is the Norian, unconformable with the Laurentian, viz., not fitting into it, as though the latter, first made under water, solidified and raised, had again been depressed and received these sec-