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Updated: June 8, 2025
Decomposing into Spherical Masses. Rude columnar Structure. Graphic Granite. Mutual Penetration of Crystals of Quartz and Feldspar. Glass Cavities in Quartz of Granite. Porphyritic, talcose, and syenitic Granite. Schorlrock and Eurite. Syenite. Connection of the Granites and Syenites with the Volcanic Rocks. Analogy in Composition of Trachyte and Granite.
Then the syenite or granite seemed as if it had been melted, for it was all in striae, which striae, as they do elsewhere, run east and west. With the change in geological structure we get a different vegetation. Instead of the laurel-leaved trees of various kinds, we have African ebonies, acacias, and mimosae: the grass is shorter and more sparse, and we can move along without wood-cutting.
FIRST, a white syenite, streaked and mottled with red; it consists of well- crystallised feldspar, numerous grains of quartz, and brilliant, though small, crystals of hornblende. The feldspar and hornblende in this and the succeeding cases have been determined by the reflecting goniometer, and the quartz by its action under the blowpipe.
The Isles of Shoals are also mostly syenite, but there are large boulders of coarse granite lying about, and in some places the syenite changes suddenly to granite as if the two had been welded together. Then there are dykes of dark brown trap or ancient lava, from four to ten feet wide running across the islands from south-west to north-east, and others again at right-angles to these.
But the discovery in 1857 of a group of stones in the white chalk near Croydon, the largest of which was syenite and weighed about forty pounds, accompanied by pebbles and fine sand like that of a beach, has been shown by Mr. Godwin Austen to be inexplicable except by the agency of floating ice.
This fine syenite, the strata of which incline to the north-west, alternates twice with serpentine. The layers of intercalated serpentine are three toises thick. Farther south, towards Regla and Guanabacoa, the syenite disappears, and the whole soil is covered with serpentine, rising in hills from thirty to forty toises high, and running from east to west.
In the district of Glatz the fissures of the gabbro are filled with a steatite of a greenish white colour, and the rock which was long thought to belong to the grunsteins* is a close mixture of feldspar and diallage. Near Guanaxuato, in Mexico, I saw it alternating with syenite.
Although there is so wide a difference in appearance between the lava of the little streams, and especially of their red scoriaceous crusts, and one of these angular ejected fragments, which at first sight might readily be mistaken for syenite, yet I believe that the lava has originated from the melting and movement of a mass of rock of absolutely similar composition with the fragments.
The Baths of Bareto are agreeable, but of nearly the same temperature as the atmosphere. The geologic constitution of this group of serpentine rocks, from its insulated position, its veins, its connection with syenite and the fact of its rising up across shell-formations, merits particular attention.
These andesitic rocks form mountain masses of a white colour, which, in their general outline and appearance in their joints in their occasionally including dark-coloured, angular fragments, apparently of some pre-existing rock and in the great dikes branching from them into the superincumbent strata, manifest a close and striking resemblance to masses of common granite and syenite: I never, however, saw in these andesitic rocks, those granitic veins of segregation which are so common in true granites.
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