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At the mines of Aristeas, the silver veins are said to be unproductive as soon as they pass into the green strata, whereas at S. Rosa, only two or three miles distant, the reverse happens; and at the time of my visit, the miners were working through a red stratum, in the hope of the vein becoming productive in the underlying green sedimentary mass.
So considerable is the solvent work that a large part of the inorganic waste appears to be taken up by the waters, so as to leave the bottom essentially without sedimentary accumulations. The sea, in a word, appears to be eating into rocks which it laid down before the depression attained its present great depth.
The dearth of calcareous matter over wide areas is characteristic of the Old Red Sandstone. This is, no doubt, in great part due to the absence of shells and corals; but why should these be so generally wanting in all sedimentary rocks the colour of which is determined by the red oxide of iron?
For, whatever has been given to one area must always have been borrowed from another; a truth which, obvious as it may seem when thus stated, must be repeatedly impressed on the student's mind, because in many geological speculations it is taken for granted that the external crust of the earth has been always growing thicker in consequence of the accumulation, period after period, of sedimentary matter, as if the new strata were not always produced at the expense of pre-existing rocks, stratified or unstratified.
On the other hand, the worthlessness of the assumption that we have discovered the earliest, or anything like the earliest, organic remains, is becoming equally clear. That the oldest known sedimentary rocks have been greatly changed by igneous action, and that still older ones have been totally transformed by it, is becoming undeniable.
But, whatever the motive, all will be in some measure benefited. None may wholly escape the good of Nature, however imperfectly exposed to her blessings. The minister will not preach a perfectly flat and sedimentary sermon after climbing a snowy peak; and the fair play and tremendous impartiality of Nature, so tellingly displayed, will surely affect the after pleadings of the lawyer.
The vast series of sedimentary rocks since the Carboniferous age, to say nothing of the volcanic, that make up these periods, are largely wanting east of the Mississippi, except in New Jersey and in some of the Gulf States. They are recent. They are like the history of our own period compared with that of Egypt and Judea.
The eastern and most important band consists of hills and mountains of ancient crystalline rocks, somewhat resembling those of the Laurentian highland but by no means so old. West of this comes a broad valley eroded for the most part in the softer portions of a highly folded series of sedimentary rocks which are of great age but younger than the crystalline rocks to the east.
These lavas did not, however, come to the surface, but as they rose they lifted the sandstones, shales, and limestones, to a thickness of 2,000 or 3,000 feet or more, into great domes. Then the molten lavas cooled in great lenses of mountain magnitude, with the sedimentary rocks domed above them.
It would, however, be a great error to assume, as some have done, that the hypogene rocks, considered as aggregates of simple minerals, are really more homogeneous in their composition than the several members of the sedimentary series.
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