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It is demonstrable that such basaltic rock as contains zeolite and calcareous spar, as most of our whin-stones do, could not have been the eruption of a volcano, consequently those rocks must have been masses protruded in a fluid state, under an immense cover of earth at the time of their production; and they could not have risen immediately out of the sea, with all their various minerals, their veins and cutters, their faces and their angles.
This great naturalist is convinced that the spar had not been here introduced by infiltration, although that is the very method which he employs to form concretions, not only of spar but of crystal, zeolite, and pyrites, in the closest cavities of the most solid rocks of basaltes.
These are the agates, calcedonies, calcareous and zeolite nodules, which are found produced in our whin-stone or subterraneous lavas, that is, the amygdaloides of Crondstedt. Naturalists explain the formation of those nodular bodies differently.
It is not possible to crystallise this alkaline salt from a dissolved state, without the combination of that quantity of water, nor to separate that water without destroying its crystalline state. But in this mineral specimen, we have a solid crystalline salt, with a structure which, upon fracture, appears to be sparry and radiated, something resembling that of zeolite.
Clarke noticed among the pebbles near the Lake of Tiberias pieces of a porous rock resembling the substance called toadstone in England; its cavities were filled with zeolite. Native gold was likewise found there, but the quantity was so small as not to draw from the travellers a suitable degree of attention.
This is also another form of igneous rock, admitting of every variety of composition. It comprehends any rock in which round or almond-shaped nodules of some mineral, such as agate, chalcedony, calcareous spar, or zeolite, are scattered through a base of wacke, basalt, greenstone, or other kind of trap. It derives its name from the Greek word amygdalon, an almond.
He described the geological structure of the lands through which we had passed as follows: The pantanals were of Pleistocene age. Along the upper Sepotuba, in the region of the rapids, there were sandstones, shales, and clays of Permian age. The rolling country east of this contained eruptive rocks a porphyritic disbase, with zeolite, quartz, and agate of Triassic age.
C| TUFA AND TRAP. In Londa, the bottom of the valley E| 2500 feet. is formed of ferruginous conglomerate on the surface; N| Lake Ngami. hardened sandstone, with madrepore holes, T| banks of gravel, and occasionally trap; R| south of 12 Degrees, large patches of soft A| TUFA. calcareous tufa, with pebbles of jasper, L| agates, &c., lie on various horizontal traps, | amygdaloids with analami and mesotype, which is P| burst through by basaltic rocks forming hills, L| and showing that the bottom of the valley A| RADIATED ZEOLITE. consists of old silurian schists; T| there are also various granitic rocks E| cropping through the trap.
In the northern parts of this group there are some cliffs of gravel and of the boulder formation. The lavas attain a thickness of from two to three hundred feet; they are extremely variable in colour and nature, being compact, or brecciated, or cellular, or amygdaloidal with zeolite, agate and bole, or porphyritic with glassy albitic feldspar.
At Bombwe we have the same trap, with radiated zeolite, probably mesotype, and it again appears at the confluence of the Chobe, farther down. As we passed up the river, the different villages of Banyeti turned out to present Sekeletu with food and skins, as their tribute.
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