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These granites, which seem formerly to have been contiguous to those of Britanny and Cornwall, are perhaps the wrecks of a chain of mountains destroyed and sunk in the waves. Large and beautiful crystals of feldspar characterise this rock. Common tin ore is sometimes discovered there, but working the mines is a laborious and unprofitable operation for the inhabitants of Galicia.
The geologists were interested, and we others learnt something of olivines, green in crystal form or oxidized to bright red, granites or granulites or quartzites, hornblende and feldspars, ferrous and ferric oxides of lava acid, basic, plutonic, igneous, eruptive schists, basalts &c. All such things I must get clearer in my mind. Tuesday afternoon a cold S.E. wind commenced and blew all night.
The work, petty as it may seem, is much too great for one man, so prodigal is Nature of her forms, in the stream as in the ocean; but what if a correspondence were opened between a few fishermen of whom one should live, say, by the Hampshire or Berkshire chalk streams; another on the slates and granites of Devon; another on the limestones of Yorkshire or Derbyshire; another among the yet earlier slates of Snowdonia, or some mountain part of Wales; and more than one among the hills of the Border and the lakes of the Highlands?
We saw gold, silver, copper, tin, and iron, as well as metals which were quite new to us. Some of these had a purple, blue, or green colour, and emitted a most agreeable fragrance. There are granites and porphyries, marbles and petrifactions of the most exquisite grain or tints.
This makes up one-quarter of the crust, leaving only one-quarter to be accounted for. Silicon mixed with oxygen makes silica or quartz. There are few rocks which have not a large amount of quartz in them. Common flint, sandstones, and the sand of our shores, are made of quartz, and therefore belong to the first class of Silicious or Flint Rocks. Granites and lavas are about one-half quartz.
Carried to greater depths and subjected to more intense heat, the strata have been completely fused, and the liquid or pasty mass, invading the contorted strata above it, has formed perfectly crystalline intrusive granites and greenstones.
The same processes of time which cause your Oxford oolite to flake away like the leaves of a mouldering book, only warm with a glow of perpetually deepening gold the marbles of Athens and Verona; and the same laws of chemical change which reduce the granites of Dartmoor to porcelain clay, bind the sands of Coventry into stones which can be built up halfway to the sky.
From this hypothesis of the great depth at which the granites originated, has been derived the name of "Plutonic rocks."
The sonority of the granites round about exaggerates the noise of the oars on the enclosed water, and there is something confusing in the thought that we are rowing and floating between the walls where formerly, and for centuries, men were used to prostrate themselves with their foreheads on the stones. And now it is quite dark; the hour grows late.
At my return from Mexico in 1807, when I showed the granites of Atures and Maypures to M. Roziere, who had travelled over the valley of Egypt, the coasts of the Red Sea, and Mount Sinai, this learned geologist pointed out to me that the primitive rocks of the little cataracts of Syene display, like the rocks of the Orinoco, a glossy surface, of a blackish-grey, or almost leaden colour, and of which some of the fragments seem coated with tar.
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