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The whole mass of the mountain consists of volcanic rock, varieties of andesite; there is no trace of a crater, nor of any fragmental materials, such as are usually ejected from a volcanic vent of eruption. Lava Crater-Cones. A third form of volcanic mountain is that which has been built up by successive eruptions of basic lava, such as basalt or dolerite, when in a molten condition.
They form a set of dikes of a fine-grained dark greenstone or dolerite, composed of feldspar and pyroxene, with occasional scales of mica and grains of pyrites. Their width varies from a few feet to a hundred yards, and they have a columnar structure, the columns being truly at right angles to the plane of the dike. Some of the dikes send off branches.
We had two lectures last week the first from Debenham dealing with General Geology and having special reference to the structures of our region. It cleared up a good many points in my mind concerning the gneissic base rocks, the Beacon sand-stone, and the dolerite intrusions. I think we shall be in a position to make fairly good field observations when we reach the southern land.
The original excessive fluidity of the Deccan trap is proved by the remarkable horizontality of the beds over large areas, and the extensive regions covered by very thin sheets of basalt or dolerite. As regards the geological age of this great volcanic series much uncertainty exists, owing to the absence of marine forms in the inter-trappean beds.
The earliest forerunners of the more recent lavas seem to have been erupted in Jurassic times, in the form of sheets of contemporaneous basalt or dolerite amongst the Antola limestones which are of this period. But the great mass of the volcanic rocks are much more recent, and may be confidently referred to the late Cretaceous or early Tertiary epochs.
It resembles clay of a yellowish or brown colour, and passes gradually from the soft state to the hard dolerite, greenstone, or other trap rock from which it has been derived.
Bowers points out a continuous dark band. Is this the dolerite sill? Thursday, November 23. Camp 19. Getting along. I think the ponies will get through; we are now 150 geographical miles from the Glacier. But it is still rather touch and go. If one or more ponies were to go rapidly down hill we might be in queer street.
Limerick in Ireland. The rocks consist chiefly of basalt, dolerite, melaphyre and felstone. Devonian Period. Volcanic rocks of Devonian age occur in the South of Scotland, consisting of felstone-porphyries and melaphyres; also at Boyle, in Roscommon, and amongst the Glengariff beds near Killarney in Ireland. Upper Silurian Period.
It stood on edge, and several dikes of basalt, with dolerite, had cut through it. Mburuma sent two men as guides to the Loangwa. These men tried to bring us to a stand, at a distance of about six miles from the village, by the notice, "Mburuma says you are to sleep under that tree." On declining to do this, we were told that we must wait at a certain village for a supply of corn.
The lava-sheets vary considerably in character, ranging from finest compact basalt to coarsely crystalline dolerite, in which olivine is abundant. The columnar structure is not prevalent, the rock being either amorphous, or weathering into concentric shells.
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