The geological features of the island are replete with interest. The formation is purely of a volcanic character and embraces every variety pertaining to that Basaltic, cellular, and tufaceous lavas abound in diversified forms. The basaltic is generally porous and scoriform, but on the slope of the hills the substrata are frequently as com pact and dense is that of the coast-line. Near Anakena may be seen hills composed of scoria quite as cellular as pumice, and in close proximity compact beds having a dark blue basis, composed of crystals of glassy feldspar and olivine. The cellular formation is mixed pumice and slag, in some cases similar
to volcanic cinder, having the lightness and qualities of coke. In some of the varieties the cavities are filled with olivine crystals partly decomposed, but generally the cavities are empty. This lava when mixed with feldspar is sometimes of gray color; not unfrequently several tints of red may be seen, though the most common is a dark, lusterless brown.
The tufaceous lavas are extremely interesting, because they form the most prominent feature in the physiognomy of the island. To this geological structure, with the incessant action of the trade-winds and heavy rains, is due the fact that the island is surrounded by precipitous cliffs, rising in some cases to a thousand feet in height. The formation is extremely friable, and by the action of the elements, enormous masses are continually disappearing beneath the waves of the sea that beat upon this unprotected shore. These tufas differ considerably in consistency at the eastern end of the island. The species is a fine light-red dust that is blown about by the wind and is destitute of vegetation; towards the southwest end the basis is a compact mud-like red clay, while the colossal crowns, intended to adorn the gigantic statues, are carved out of a variety that has been scorified in one of the craters, and is of a dull reddish color.
The ordinary rules for estimating the age of rocks by compactness can be applied at Easter Island only hypothetically, because the scoriform and more dense specimens are found immediately contiguous to one another. In places they are quite conglomerated, as though older formations had been disturbed by volcanic convulsions, while a new flow of lava enveloped and sealed the whole into a heterogeneous mass. During our short stay on the islands there was no opportunity to measure the lava flow or to make investigations of that nature.
Natural caves are numerous, both on the coast-line and in the interior of the island. Some of them are of undoubted antiquity and bear evidence of having been used by the early inhabitants as dwellings and as burial places. It is reported that small images, inscribed tablets, and other objects of interest have been hidden away in such caves and finally lost through land-slides.
The numerous bills on this island have gently sloping sides, except where they approach the coast, falling at this point precipitously to the sea. The plains are irregularly shaped, and some of the smaller ones rise to a considerable height. The physical character of the soil is alluvial. The substratum is volcanic ash and stones, and the upper formation is composed of decayed vegetable matter mingled with a rich deposit of decomposed lava washed down from hills by the frequent rains. These plains being formed by the periodical eruptions of the volcanoes, some difference may be noted in the quantity of the soil, varying according to location.
After the successive discharges of lava from the craters of Rana Roraka and Rana-kao had prescribed the limits of the island and when
this flow had ceased, there was a heavy deposit of mud, covering deeply both hill and dale. This condensed earth, after the lapse of centuries, has formed a soil that produces a natural grass affording an excellent pasturage for flocks and herds. The expiring energy of the volcanic power appears to have been directed, long after the formation of this soil, to sprinkling thickly the entire surface of the island with stones and small bowlders, thus providing the means of attraction and holding the moisture, nature's substitute, as it were, for trees. The natives have distinct names for the following varieties: Black and red tufa with volcanic cinder and pumice are called "Maea-Hane-hane," "maea" being the generic term applied to all stone. A soft gray tufa is ground down with the juice of the sugar-cane and used as a paint. This is known as "Kiri-kiri Teu." Hard slates, black, red, and gray, are used for stone axes and called "Maea-Toke." Granite used for the same purpose is known as "Maea-Nevhive." The hardest and finest stone implements are made of the flinty beach pebble known as "Maea-Rengrengo." The hard cellular stories from which the majority of the platforms are built are called "Maea-Pupura." The material from which images were constructed is called "Maea-Matariki," and the obsidian from which spear-heads were made is known as "Maea-Mataa."