It required some time for the party to set out on the ride from Assouan, as there was a good deal of difficulty in getting everybody comfortably seated. As we have before stated, Frank and Fred selected their camels before breakfast, and the Doctor did not take many minutes for making his choice. The three set out in advance of the rest, and proceeded to the quarries that furnished the stone for the obelisks, the coffins of the sacred bulls, and many other things that have become famous in the history of ancient Egypt. In the quarries is an obelisk partly finished, but not completely detached from its bed. According to measurements, it would have been ninety-five feet long and eleven feet square at the base. Nobody can tell by what king it was ordered, or why it was never finished and removed. A crack extends across it, but the general belief is that it was made long after the abandonment of the work. A little distance from the quarries is a large coffin which became injured during its removal, and was consequently given up. The stone is of that peculiar red granite known as sienite, and admits of a very high polish. In response to a question by one of the youths, the Doctor said that ordinary granite is composed of mica, felspar, and quartz, while in sienite there is little or no mica, and its place is filled by hornblende. Sienite is harder than most of the other granites, and this quality, combined with its color, causes it to be preferred for ornamental work. He farther remarked that the rocks around Assouan are not exclusively sienite; on the contrary, they are mostly true granite, with occasional variations of porphyry. Some geologists assert that four or five kinds of rock may be found there, and interesting specimens may be gathered for mineralogical cabinets. The process of quarrying among the ancients was easy to comprehend, owing to the unfinished state of the obelisk to which we have referred. A crevice or trench was cut in the rock, and then wedges of dry wood were driven in; water was applied to the wedges; the wood swelled, and finally its great expansive force caused the rock to split asunder. It was slow work, but generally sure. The same plan is still in use in some parts of India, and the stones for the construction of King Solomon's Temple at Jerusalem were quarried in the same way. From the quarries the ride was continued to the bank of the Nile, opposite the island of Philæ. The boys were elated to think they were really in Nubia, a country of which they had read and heard, but considered so far away that they were not very likely to see it. They had crossed the boundary between Egypt and Nubia, and, by a free use of their imaginations, found no great difficulty in placing themselves in Central Africa. [Illustration: VIEW OF PHILÆ FROM THE HEAD OF THE CATARACT.]