Finally, the thing about the geology of Africa that strikes one as especially significant is that throughout this vast area, just opening up to science, there is nothing new--no unknown force at work; no rock strange to the petrographer; no pause in denudation; no formation, texture, or structure to put the law of continuity to confusion. Rapid radiation, certainly, replaces the effects of frost in northern lands--and the enormous denudation due to this cause is a most striking feature of tropical geology. The labors of the worm, again, in transporting soil in temperate climates are undertaken by the termite; but here, as elsewhere, every fresh investigation tends to establish more and more the oneness and simplicity of Nature. IX. A POLITICAL WARNING. When I reached the coast to embark for England after my wanderings in the interior, the Portuguese authorities at Quilimane presented me with various official documents, which I was told I must acknowledge with signatures and money before being permitted to leave Africa. Having already had to pay certain moneys to Portugal to get into this country, it was a shock to find that I had also to pay to get out; but, as no tax could be considered excessive that would facilitate one's leaving even the least of the Portuguese East African colonies, I cheerfully counted out the price of my release. Before completing the conveyance, however, my eye fell on six words prominently endorsed on one of the documents, which instantly tightened my purse-strings. The words were, "TAX FOR RESIDING IN THE INTERIOR"--so much. Now a day or two spent in waiting for a steamer could scarcely be construed into residence, nor could a strip of coast-line with propriety be termed the interior, so I ventured to point out the irrelevancy to the Portuguese official. Waiving the merely philological question of residence, he went at once to the root of the matter by informing me that the Portuguese definition of the word Interior differed materially from that of England. The Interior, he said, comprised the whole of Africa inland from the coast-province of Mozambique, and included, among other and larger possessions, the trifling territories of the Upper Shiré Highlands, Lake Shirwa, and Lake Nyassa. These last, he assured me, belonged to Portugal, and it became me, having therein shared the protection of that ancient flag, to acknowledge the obligation to the extent of so many hundred Reis.