BLACKNESS OF THE ROCKS—^ABRASION BY SAND. 
231 
of silicified wood. A pepple of white quartz had a red surface, looking as if it had heen painted. 
A peculiar ‘ 4 inky blackness” of the rocks around the Egyptian Deserts has heen noticed by 
travellers, and is, doubtless, similar in its origin. Humboldt observed that the granite along 
the Orinoco had acquired a grayish-black coating wherever it had heen in contact with water, 1 
and that it was not found along those rivers which have black or coffee-brown waters. The 
source of the coloring is thus indicated to us, and it may he that the rocks along the Colorado, 
and the pebbles of the Desert, received, during their submergence, a coating of organic matter, 
which, under the burning rays of the sun in that almost rainless region, has become a perfect 
lacquer. A similar discoloration was observed within the limits of the Great Basin, on the 
hanks of the Mojave, several feet above the present reach of the stream. The outcrops of gneiss 
were so black that the rock could not be recognized without breaking it. 
The wonderful abrading effects produced by the moving sand in the lower part of the Pass of 
San Bernardino, near the margin of the Desert, are described in Chapter VIII. A figure is 
also given illustrating the effect of the sand upon vertical surfaces of the rock, where it was 
composed of minerals differing in their hardness, as in quartz and feldspar. The effect of this 
driving sand is, however, much more vividly shown at the locality by the grooved and polished 
surfaces of the rock. Specimens of the rock, abraded and smoothed in this manner were 
GRANITE FURROWED AND POLISHED BY MOVING SAND. 
obtained, and the attempt has heen made to represent the surface by a figure, hut with indif¬ 
ferent success. The number of small grooves and channels on the sides of the principal grooves 
could not he fully represented. 2 
1 I find the following note upon this subject in Humboldt’s Views of Nature, Bohn’s Edit., 1850, page 141. In the Orinoco, 
and more especially at the cataracts of Maypures and Atures, (not in the Black River or Rio Negro,) all blocks of granite, 
even pieces of white quartz, wherever they come in contact with the water, acquire a grayish-black coating, which does not 
penetrate 0.01 of a line into the interim of the rock. The traveller might suppose that he was looking at basalt or fossils, 
colored with graphite. Indeed, the crust does actually appear to contain manganese and carbon. Isay “ appears ” to do so, 
because the phenomenon has not been thoroughly investigated. Something perfectly analogous to this was observed by 
Rozier, in the syenitic rocks of the Nile, (near Syene and Phil® ;) by the unfortunate Captain Tuckey, on the rocky banks of 
the Saire ; and by Sir Robert Schomburgk, at Berbice. On the Orinoco these leaden-colored rocks are supposed, when wet, to 
give forth noxious exhalations ; and their vicinity is believed to be conducive to the generation of fevers. It is also remarkable 
that the South American rivers generally, which have black waters, (aguns negras,) or waters of a coffee-brown or wine-yellow 
tint, do not darken the granite rocks ; that is to say, they do not act upon the stone in such a manner as to form, from its 
constituent parts, a black or leaden-colored crust. 
* This engraving was m t received in season to be inserted in Chapter VIII. 
