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It is interesting thus to trace the steps by which a compact granular rock becomes converted into a vesicular, pseudo-porphyritic lava, and finally into red scoriae.
They were often hid from the sight by a quantity of scoriae that had formed a crust over them; and the lava, having been conveyed in a covered way for some yards, came out fresh again into an open channel.
Hartung and myself in 1853, at the height of 1000 feet above the level of the sea, crops out at the base of a cliff formed by the erosion of a gorge cut through alternating layers of basalt and scoriae, the product of a vast succession of eruptions of unknown date, piled up to a thickness of 1000 feet, and which were all poured out after the plants, of which about twenty species have been recognised, flourished in Madeira.
But on examining the walls of the crater we find precipices of sandstone and shale which exhibit no signs of the action of heat; and we look in vain for those beds of lava and scoriae, dipping outward on every side, which we have been accustomed to consider as characteristic of volcanic vents.
A hill formed of the older series of volcanic rocks, and which has been incorrectly considered as the crater of a volcano, is remarkable from its broad, slightly hollowed, and circular summit having been filled up with many successive layers of ashes and fine scoriae.
The lava carried vast masses of burnt stone and sulphur on its surface, like dross on melted lead, and nothing was visible toward Bosco Trecase but endless acres of dark scoriae, broken here and there by the greenish, curling smoke of sulphur. At one point a great cone pine tree, torn up by its roots and turned to black charcoal, stuck out of the mass at a sharp angle.
It appears from the investigations of Sir R. Murchison in Shropshire, that when the Lower Silurian strata of that country were accumulating, there were frequent volcanic eruptions beneath the sea; and the ashes and scoriae then ejected gave rise to a peculiar kind of tufaceous sandstone or grit, dissimilar to the other rocks of the Silurian series, and only observable in places where syenitic and other trap-rocks protrude.
Lava-currents have also flowed upon the land and along the bottom of the sea volcanic sand, pumice, and scoriae have been showered down so abundantly that whole cities were buried- tracts of the sea have been filled up or converted into shoals and tufaceous sediment has been transported by rivers and land-floods to the sea.
From the summit of these solitary regions our eyes wandered over an inhabited world; we enjoyed the striking contrast between the bare sides of the peak, its steep declivities covered with scoriae, its elevated plains destitute of vegetation, and the smiling aspect of the cultured country beneath.
The patent further describes that "as the workman stirs the metal," the scoriae will separate, "and the particles of iron will adhere, which particles the workman must collect or gather into a mass or lump." This mass or lump was then to be raised to a white heat, and forged into malleable iron at the forge-hammer.
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