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Updated: May 24, 2025
The earlier eruptions seem to have been chiefly granitic, though this must not be understood in too wide a sense, since there are granite formations even as late as the Tertiary period; those of the middle periods were mostly porphyries and basalts; while in the more recent ones, lavas predominate.
It is of course possible that the Leadville porphyries are only phases of rocks rich in silver, lead, and iron, which underlie this region, and which have been fused and forced to the surface by an ascending mass of deeper seated igneous rock; but even if the argentiferous character of the porphyry shall be proved, it will not be proved that such portions of it as here lie upon the limestone have furnished the ore by the descending percolation of cold surface waters.
This earthy substance often becomes coarser, and contains minute rounded fragments of porphyries and rounded grains of quartz, and in one case so many of the latter as to resemble a common sandstone.
The table of Serpentine is here: also the table inlaid with porphyries; one with a series of bivalve shells ; and in the centre of the room is the stalagmitic table, from the Blythe lead mine, Derbyshire, with black marble legs from Bakewell, given to the trustees of the Museum by the Duke of Rutland.
I am tempted to believe, that where the clay-slate has been metamorphosed at great depths beneath the surface, gneiss, mica- slate, and other allied rocks have been formed, but where the action has taken place nearer the surface, feldspathic porphyries, greenstones, etc., have resulted, often accompanied by submarine volcanic eruptions.
But some of these same porphyries are partially unconformable, as b, and may lead us to suspect that the others also, notwithstanding their appearance of interstratification, have been forcibly injected. Some of the porphyritic rocks above mentioned are highly quartzose, others very feldspathic.
Near our camp, a dyke or wall of the aspect of a flinty red conglomerate, crossed the river from south-west to north-east. I believe that this rock belongs to the porphyries of Glendon, and of the upper Gloucester. We continued to feel the breeze, or rather a puff of wind, between 7 and 8 o'clock at night; it was often very strong and cold, and prevented the mosquitoes from molesting us.
The coarse granite of Harney Peak indicating that, as the central point of the earliest upheaval, and the three porphyries known as rhyolite, trachyte, and phonolite, showing the uplifts of later periods to have had their centers a little more to the north, but the entire area is said to be only about sixty miles long and twenty-five miles in width.
The pebbles consist chiefly of the brick-red porphyry just described and of white granite, both probably derived from the ranges to the west, and of altered clay-slate and of certain porphyries, apparently belonging to the rocks of the Uspallata chain.
Beds of this nature, alternating with numerous compact and amygdaloidal porphyries, which have flowed as submarine lavas, and associated with great mountain- masses of various, injected, non-stratified porphyries, are prolonged the whole distance up to the Cumbre or central ridge.
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