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There is some fine-grained syenitic granite, orbicularly marked by ferruginous lines, and weathering into vertical, cylindrical holes, almost touching each other. In the gneiss, concretions of granular feldspar and others of garnets with mica occur.
In the little stream were many pebbles of vesicular trap, probably an amygdaloid with the kernels decomposed, but containing particles of olivine; also pebbles of a syenitic compound, consisting of quartz, hornblende, and felspar; and of compact felspar, mottled green and white, the green colour probably being due to chlorite or green earth, and they enclosed also decomposed crystals of mica and hornblende.
The quadruple compound of quartz, feldspar, mica, and hornblende, may be termed Syenitic Granite, and forms a passage between the granites and the syenites. This rock occurs in Scotland and in Guernsey. We now come to the second division of the Plutonic rocks, or those having less than 60 per cent of silica, and which, as before stated, are usually called syenitic.
Having observed, for example, that the syenitic granite of Norway, in which the mineral called zircon abounds, has altered the Silurian strata wherever it is in contact, we do not hesitate to refer other masses of the same zircon-syenite in the south of Norway to a post- Silurian date.
The southern part of this island is mainly formed of mountains of greenstone, which often assumes a syenitic character, and contains much hypersthene. These mountains, in their lower half, are generally encased by strata containing numerous small corals and some shells. These shells have been examined by Mr.
In regard to the age of the gold of the Ural mountains, in Russia, which, like that of California, is obtained chiefly from auriferous alluvium, it occurs in veins of quartz in the schistose and granitic rocks of that chain, and is supposed by Sir R. Murchison, MM. Deverneuil and Keyserling to be newer than the syenitic granite of the Ural perhaps of tertiary date.
In the southern extremity of Norway there is a large district, on the west side of the fiord of Christiania, which I visited in 1837 with the late Professor Keilhau, in which syenitic granite protrudes in mountain masses through fossiliferous strata, and usually sends veins into them at the point of contact.
Of this nature, among others, is a remarkable angular block of syenitic greenstone, 4 1/2 feet by 4 feet square, and 2 feet thick, which Mr. In Ireland we encounter the same difficulty as in Scotland in determining how much of the glaciation of the higher mountains should be referred to land glaciers, and how much to floating ice, during submergence.
The 11th still saw me plunging through the grass, which showered drops of rain on me every time I made a step forward. In two hours we crossed a small stream, with slippery syenitic rocks in its bed, showing the action of furious torrents. Mushrooms were in abundance, and very large.
It appears from the investigations of Sir R. Murchison in Shropshire, that when the Lower Silurian strata of that country were accumulating, there were frequent volcanic eruptions beneath the sea; and the ashes and scoriae then ejected gave rise to a peculiar kind of tufaceous sandstone or grit, dissimilar to the other rocks of the Silurian series, and only observable in places where syenitic and other trap-rocks protrude.
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