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These rocks consisted of quartzose and schistose beds, the first of which supplied white sand and the other argillaceous mud to a river which entered the sea at this point, carrying down in its turbid waters much drift-wood and the leaves of plants.
We first stopped at a spring issuing, not from the calcareous rock, but from a layer of quartzose sandstone. The temperature was 21 degrees, consequently 1.5 degrees less than the spring of Quetepe; and the difference of the level is nearly 220 toises. Wherever the sandstone appears above ground the soil is level, and constitutes as it were small platforms, succeeding each other like steps.
There were also pebbles of a quartzose conglomerate, and others of decomposed porphyry, the base consisting of granular felspar, with crystals of common felspar. It is not improbable that good millstones might be obtained from the range of Nundewar. The grass was fortunately much better here than at the last camp. February 20.
Limestones are rare in this series, but one band of 300 feet in thickness has been traced for considerable distances to the north of Lake Huron. Beds of greenstone are intercalated conformably with the quartzose and argillaceous members of this series. No organic remains have yet been found in any of the beds, which are about 18,000 feet thick, and rest unconformably on the Laurentian rocks.
"The dune sands" of different characters, says Bremontier, "partake of the nature of the different materials which compose them. At certain points on the coast of Normandy they are found to be purely calcareous; they are of mixed composition on the shores of Brittany and Saintonge, and generally quartzose between the mouth of the Gironde and that of the Adour."
It appears from M. Parchappe's observations, that at Tandil there is a range of quartzose gneiss, very like the rocks of the S. Larga near Maldonado, running in the same N.N.E. and S.S.W. direction; so that the framework of the country here is very similar to that on the northern shore of the Plata. Geolog. page 46.
In ascending the valley, the gravel gradually becomes entirely altered in character: high up, we have pebbles of crystalline feldspathic rocks, compact clay-slate, quartzose schists, and pale-coloured porphyries; these rocks, judging both from the gigantic boulders in the surface and from some small pebbles embedded beneath 700 feet in thickness of the old tertiary strata, are the prevailing kinds in this part of the Cordillera; pebbles of basalt from the neighbouring streams of basaltic lava are also numerous; there are few or none of the reddish or of the gallstone-yellow porphyries so common near the coast.
Continuing on the same line I crossed another small watercourse falling north-east; and beyond it were hills of mica-schist and quartz, which sloped rather boldly to the southward. We then entered one of the finest tracts of forest land I ever saw. It was there three miles in width, and bounded on the south by another low hill of quartzose gravel, the soil of which was indifferent.
This conflict is very violent, but far from being so dangerous as Father Gumilla describes. We passed the Punta Curiquima, which is an isolated mass of quartzose granite, a small promontory composed of rounded blocks. There, on the right bank of the Orinoco, Father Rotella founded, in the time of the Jesuits, a Mission of the Palenka and Viriviri or Guire Indians.
Bunter-sandstein: Gres bigarre: Sandstone and quartzose conglomerate. Equisetites columnaris. The first of these, or the Keuper, underlying the beds before described as Rhaetic, attains in Wurtemberg a thickness of about 1000 feet. It is divided by Alberti into sandstone, gypsum, and carbonaceous clay-slate.
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