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Updated: June 8, 2025
The trap-rocks alluded to consist chiefly of feldspathic porphyry and amygdaloid, the kernels of the latter being sometimes calcareous, often chalcedonic, and forming beautiful agates. We meet also with claystone, greenstone, compact feldspar, and tuff.
Mr. Collins found the bank composed of amygdaloid sand, decomposed rock and sandstone, with many traces of iron. On the beach were chalcedony, cornelian, and agate. Two veins of coal have been traced in the cliff, and it is thought a large deposit exists there. The natives have a story that the cliff smokes whenever a human being approaches it, but I saw no indications of smoke as I passed.
The newest of these, connected with the higher series of coal- measures, is well exhibited along the shores of the Forth, in Fifeshire, where they consist of basalt with olivine, amygdaloid, greenstone, wacke, and tuff.
M. Rose has described an amygdaloid, brought by Dr. In some of these intrusive masses, the porphyries exhibit, more or less plainly, a brecciated structure, like that often seen in volcanic masses.
Neither olivine nor amphibole can be distinguished. Before it shows itself as a separate stratum, rising in small conic hills, the amygdaloid seems to alternate by layers with the diorite, which we have mentioned above as mixed with carburetted slate and amphibolic serpentine.
The tops of the hills are often crowned with basaltic columns, and a soft porous amygdaloid abounds on the outskirts of the Mexican valley. Besides this, traces of more recent volcanic action abound, in the shape of numerous extinct craters in the high plateaus, and immense "pedrigals" or fields of lava not yet old enough for their surface to have been disintegrated into soil.
Here the difficulty seems to arise from the too intimate and too numerous relations observed in rocks that are thought not to belong to the same family. I found no real basalt or dolerite; but the presence of pyroxene in the amygdaloid of Parapara leaves little doubt of the igneous origin of those spheroidal masses, fissured, and full of cavities.
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