o not
appear in the sketch. The chief rock is syenite, some portions of which
have a beautiful blue tinge like _lapis lazuli_ diffused through them;
others are grey. Blocks of granite also abound, of a pinkish tinge; and
these with metamorphic rocks, contorted, twisted, and thrown into every
conceivable position, afford a picture of dislocation or unconformability
which would gladden a geological lecturer's heart; but at high flood this
rough channel is all smoothed over, and it then conforms well with the
river below it, which is half a mile wide. In the dry season the stream
runs at the bottom of a narrow and deep groove, whose sides are polished
and fluted by the boiling action of the water in flood, like the rims of
ancient Eastern wells by the draw-ropes. The breadth of the groove is
often not more than from forty to sixty yards, and it has some sharp
turnings, double channels, and little cataracts in it. As we steamed up,
the masts of the "Ma Robert," though some thirty feet high, did not reach
the level of the flood-channel above, and the man in the chains sung out,
"No bottom at ten fathoms." Huge pot-holes, as large as draw-wells, had
been worn in the sides, and were so deep that in some instances, when
protected from the sun by overhanging boulders, the water in them was
quite cool. Some of these holes had been worn right through, and only
the side next the rock remained; while the sides of the groove of the
flood-channel were polished as smooth as if they had gone through the
granite-mills of Aberdeen. The pressure of the water must be enormous to
produce this polish. It had wedged round pebbles into chinks and
crannies of the rocks so firmly that, though they looked quite loose,
they could not be moved except with a hammer. The mighty power of the
water here seen gave us an idea of what is going on in thousands of
cataracts in the world. All the information we had been able to obtain
from our Portuguese friends amounted to this, that some three or four
detached rocks jutted out of the river in Kebrabasa, which, though
dangerous to the cumbersome native canoes, could be easily passed by a
steamer, and that if one or two of these obstructions were blasted away
with gunpowder, no difficulty would hereafter be experienced. After we
had painfully explored seven or eight miles of the rapid, we returned to
the vessel satisfied that much greater labour was requisite for the mere
examination of the cataract