While the formation of lava stalactites of such perfection and beauty is peculiar to the Hawaiian lava tunnels, the formation of the tunnel in connection with lava outflow is the rule wherever a dissipation at the end has permitted of drainage. A few hours only after the flow has begun, the frozen surface has usually a thickness of a few inches, and this cover may be walked over with the lava still molten below. At first in part supported by the molten lava, the tunnel roof sometimes caves in so soon as drainage has occurred. [Illustration: FIG. 109.—Surface of lava of the Pahoehoe type.] Wherever basaltic lava has spread out in valleys on the surface of more easily eroded material, either cinder or sedimentary formations, the softer intervening ridges are first carried away by the eroding agencies, leaving the lava as cappings upon residual elevations. Thus are derived a type of table mountain or _mesa_ of the sort well illustrated upon the western slopes of the Sierra Nevadas in California (Fig. 108). [Illustration: FIG. 110.—Three successive views to illustrate the growth of the Island of Savaii from the outflow of lava at Matavanu in the year 1906. _a_, near the beginning of the outflow; _b_, some weeks later than _a_; _c_, some weeks later than _b_ (after H. I. Jensen).] The surface which flowing lava assumes, while subject to considerable variation, may yet be classified into two rather distinct types. On the one hand there is the billowy surface in which ellipsoidal or kidney-shaped masses, each with dimensions of from one to several feet, lie merged in one another, not unlike an irregular collection of sofa pillows. This type of lava has become known as the _Pahoehoe_, from the Hawaiian occurrence (Fig. 109). A variation from this type is the “corded” or “ropy” lava, the surface of which much resembles rope as it is coiled along the deck of a vessel, the coils being here the lines of scum or scoriæ arranged in this manner by the currents at the surface of the stream (Fig. 123, p. 124). A quite different type is the block lava (_Aa_ type) which usually has a ragged scoriaceous surface and consists of more or less separate fragments of cooled lava (Fig. 131, p. 130). Wherever lava flows into the sea in quantity, it extends the margin of the shore, often by considerable areas. The outflow of Kilauea in 1840 extended the shore of Hawaii outward for the distance of a quarter of a mile, and a more recent illustration of such extension of land masses is furnished by Fig. 110. CHAPTER X THE RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE VOLCANIC MOUNTAINS OF EJECTED MATERIALS