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And now, away at sea and leagues from the coast they were approaching, vast islands disclosed themselves suddenly through the sea haze, standing like giants waist deep in the ocean, whilst the coast itself with its cliffs and rocks of black basalt and dolerite shewed clear, extraordinarily clear, with every detail defined in the sunlight, from the rifts in the basalt to the gulls blowing about in legions and the great sea-geese hovering and fishing.
The lower portion of the mountain is formed of bedded basalt, or dolerite with numerous dykes and veins of basalt, felstone, and pitchstone; the upper cliff, or Scuir, is composed of pitchstone of newer age, the remnant of a lava flow which once filled a river channel in the basaltic sheets. A dyke, or sheet, of porphyry is seen to be interposed between the Scuir and the basaltic sheets.
"Then of course we'll bring out a tent and camp kit for you tomorrow. Genevieve and I can ride or drive up to the waterhole each day, to picnic with you." "It will be delightful," agreed Mrs. Blake. "You ride on ahead and wait for us in the shade," said her husband. "We'll knock off for the day when we reach that dolerite dike above the waterhole. If you are ready, Ashton, we'll peg along."
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.
Sometimes we seem to see serpentine, sometimes grunstein, and sometimes dolerite and basalt. The arrangement of these problematical masses is not less extraordinary. Between San Juan, Malpaso, and Piedras Azules, they form strata parallel to each other; and dipping regularly northward at an angle of 40 or 50 degrees, they cover even the green slates in concordant stratification.
Primitive formations are nowhere seen above ground; we find only what belongs unquestionably to volcanoes: feldspar-lava, dolerite, basalt, conglomerated scoriae, tufa, and pumice-stone. Among the limestone formations we must distinguish those which are essentially subordinate to volcanic tufas* from those which appear to be the work of madrepores and other zoophytes.
Wilson did a large amount of sketching on the Beardmore his sketches, besides being wonderful works of art, helped us very much in our surveys. Fringing the great glittering river of ice were dark granite and dolerite hills, some were snow-clad and some quite bare, for their steepness resisted the white cloak of this freezing clime.
These lavas consisting of basalt and dolerite, with their varieties, and extruded from vents and fissures, spread themselves in all directions over the pre-existing lake deposits or the older sheets of augitic lava, and probably entirely buried the trachytic hills. These later sheets solidified into more solid masses than those of the second stage.
Inland, cañons of night and shoulders of dolerite and plains where nothing grew leading to great level bastions, fortifications that seemed built by rule and plumb line, with the markings of the basalt visible through the clear air. Basalt has that terrible peculiarity. It seems the work of a hand, it makes castles and fortifications whose ruled markings bear the inevitable suggestions of masonry.
The felspathic lavas, with their associated beds of tuff and breccia, rise into some of the grandest mountain crests of North Wales, such as those of Cader Idris, Aran Mowddwy, Arenig and Moel Wyn. A similar series is also represented in Ireland, ranging from Wicklow to Waterford, forming a double group of felstones, porphyries, breccias, and ash-beds, with dykes of basalt and dolerite.
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