Having completed observations at 2.10, steered 300 degrees along the foot of a range of trap hills; at 3.50 passed a dry salt lake on our right, and at 5.15 bivouacked on the side of a trap hill, among some fine oat-grass growing on calcareous tufa.

I soon came to the valley, the sides of which are composed of what is called, in the language of geology, tufa, and in that of the country, dukstein, or trass.

While I am on the subject of volcanic phenomena, I may as well add a word on the origin of the trass or tufa, which is so thickly spread over this country.

The latter consists of almost horizontal banks of tufa, from eight to twelve inches in thickness.

In an hour after our departure we reached the little island of Majava, which consists of perpendicular strata of a hard, fine-grained, volcanic tufa, with small, bright crystals of hornblende.

The country around Manila is composed of tufa of a light gray color, which being soft and easily worked, is employed as the common building material in the city.

This tufa forms one of the remarkable features of the volcanoes of the Philippine Islands, showing a strong contrast between them and those of the Pacific isles, which have ejected little else than lava and scoria.

The mountain is composed of trachytic rocks and tufa, which are occasionally seen to break through the rich and deep soil, showing themselves here and there, in the deep valleys which former volcanic action has created, and which have destroyed the regular outline of the cone-shaped mountain.

The tufa is generally found to form the gently-sloping plains that surround these mountains, and has in all probability been ejected from them.

The tufa is likewise exposed to view on the shores of the lake; but elsewhere, except on a few bare hills, it is entirely covered with the dense and luxuriant foliage.

The tufa is generally of a soft character, crumbling in the fingers, and in it are found coarse and fine fragments of scoria, pumice, etc.

The rocks in the vicinity are all tufa, and some of the springs break out close to the cold water of the lake.

These ashes had been converted by time into a soft, or friable rock, composing a stone that is called tufa.

It is that of drainage by cavities, applied by the Etruscans, Latins, and Volsci to all the Roman hills formed of volcanic tufa, the tradition of which I have found still preserved in some countries of the Abruzzi.

On his right stands a high, rocky island of dark tufa, rendered gay, amid all its magnificent formations, by smiling vineyards and teeming villages, and interesting by ruins that commemorate events as remote as the Caesars.

Returning to the shore, which now begins to incline more westwardly, we come to another swell of tufa, which has all the characteristic fertility and abruptness of that peculiar formation, a vast and populous town of near half a million of souls being seated, in nearly equal parts, on the limits of the plain and along the margin of the water, or on the hill-sides, climbing to their summits.

" [Footnote D: The volcanic rocks are the Tufa litoide, very hard, and used for paving and other such purposes; difficult to be quarried, and unfit for graves on account of this difficulty.

The third is the Pura pozzolana,which is the Tufa granulare in a state of compact sand, yielding to the print of the heel, dug like sand, and used extensively in the unsurpassed mortar of the Roman buildings.

In such cases the tufa may have been broken down into the condition of sand for removal.

In later times, as the catacombs were extended, the tufa dug out from one passage was carried into the old passages no longer used; and thus, as the catacomb extended in one direction, it was closed up in another, and the ancient graves were concealed.

And sometimes, instead of the simple loculus, or coffin-like excavation, there is an arch cut out of the tufa, and sunk back over the whole depth of the grave, the outer side of which is not cut away, so that, instead of being closed in front by a perpendicular slab of marble or by tiles, it is covered on the top by a horizontal slab.

At this point, the bed of compact tufa, in which the graves are dug, degenerates into friable and loosely compacted volcanic sand,and it was here, very probably, that the cemetery was begun, at a time when every precaution had to be used by the Christians to prevent the discovery of their burial-places.

The stratum of pozzolana at the Catacombs of St. Agnes overlies a portion of the more solid stratum of tufa, and the entrance to the sand-pit from the cemetery is by steps leading up from the end of a long gallery.

Such an entrance could have been easily concealed; and the tufa cut out for the graves, after having been reduced to the condition of pozzolana, might easily at night have been brought up to the floor of the pit.

This is of volcanic origin, and consists of a stratified earthy tufa and volcanic conglomerate, hollowed out below by the sea, succeeded by a harder vesicular rock above which one of the forms of lava has been poured out.