Here, and in the vicinity, laborers are daily employed in blowing up the rocks with gunpowder, by which process vast fragments are frequently thrown down, and repeatedly strike the precipice with a dreadful crash, which, combined with the loud report of the explosion, reëchoed from side to side by the lofty cliffs, makes a noise resembling thunder, for which it is frequently mistaken by strangers. It is the opinion of the greater part of those who have viewed these rocks, that they were once united, and were separated by some terrible convulsion of nature. A bridge of one arch, from rock to rock, over the Avon, has long been in contemplation; but if the blowing up of these rocks should still be persisted in, the design will be rendered impracticable. This is the more to be regretted, because stone of the same quality is to be procured lower down the river. Now it is in the fissures and cavities of these rocks, that the beautiful crystals called _Bristol stones_, or _diamonds_, already mentioned, are found. They are clear and brilliant, and being without color, so richly and brilliantly reflect the light, as to be almost next to the diamond in appearance, and are often palmed off on the unpracticed for the latter gem. They are extensively used in many of the plainer kinds of jewelry, and when set over gold-leaf, or thin paper of delicate tinges, are often made closely to resemble some of the richest gems known. GOLD AND SILVER MINES.[4] The mines of La Plata, so denominated on account of the abundance of silver they contain, are chiefly situated in the provinces which were formerly attached to the Spanish viceroyalty of Rio de la Plata; but which, since the South American states revolted from the mother country, have been included in the republic of Bolivia, or Upper Peru as it is sometimes called. With the exception of Mexico, Bolivia is the richest country in silver which has yet been discovered, and contains innumerable mines both of that metal and of gold. All its northern provinces teem with mineral opulence; and those of Laricaja and Carabaya, have been distinguished by the production of the latter, and still nobler metal, in its virgin state. In consequence, however, of the recent political convulsions, mining, once the richest source of revenue, is in a depressed state; many of the mines being filled with water and totally neglected. Footnote 4: The account of the mines of South America and Mexico is mostly from Humboldt, and as will be obvious to the reader, has reference in many things to their past history and progress, rather than to their present condition.