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These tuffs of the southern shores of the Grand Canary, containing the Upper Miocene shells, appear to be about the same age as the most ancient volcanic rocks of the island, composed of slaty diabase, phonolite, and trachyte.
It is therefore possible that the stones were erratic blocks found near Stonehenge. Between the horseshoe and the outer circle another circle of diabase stones is sometimes said to have existed, but very little of it now remains. The whole building is surrounded by a rampart of earth several feet high, forming a circle about 300 feet in diameter.
Submarine rocks probably exist near the coast, which furnish these very hard masses. I have compared them in my journal to the PATERLESTEIN of Fichtelberg, in Franconia, which is also a diabase, but so fusible, that glass buttons are made of it, which are employed in the slave-trade on the coast of Guinea.
LABRADOR- ROCK is a term used for a compound of labradorite or labrador-feldspar and hypersthene; when the hypersthene predominates it is sometimes known under the name of HYPERSTHENE-ROCK. GABBRO and DIABASE are rocks mainly composed of triclinic feldspars and diallage.
I have already stated that small landslips are of frequent occurrence on the sides of the hills. We had several times the entrance to our mines temporarily closed by them in the wet season. Mr. In North Wales we have, firstly, an intrusion of diabase, occurring in great mountain masses; secondly, Irregular tortuous dykes of diabase; thirdly, Elvan dykes; and, lastly, auriferous quartz veins.
This division corresponds with that proposed by Bunsen and Durocher for volcanic rocks, upon the results of analyses of a large number of specimens from various districts. Examples: Basalt, Dolerite, Hornblende rock, Diorite, Diabase, Gabbro, Melaphyre, and Leucite lava. Examples: Trachyte, Rhyolite, Obsidian, Domite, Felsite, Quartz-porphyry, Granite.
Within the horseshoe and close up to it stand the famous blue-stones, now twelve in number, but originally perhaps more. These stones are not so high as the trilithons, the tallest reaching only 7-1/2 feet. They are nearly all of porphyritic diabase.