flow, we shall find that they present equally marked differences in the character of their surfaces. The lavacurrent of 1858 was a remarkable example of a slowflowing stream, and its surface, as will be seen in fig. 22, which is taken from a photograph, has a very marked and peculiar character. A tenacious crust seems to have formed on the surface, and by the further motion of the mass this crust or scum has been wrinkled and folded in a very remarkable manner. Sometimes this folded and twisted crust presents a striking resemblance to coils of rope. Precisely similar appearances may be observed on the surface of many artificial slags when they flow from furnaces, and are seen to be due to the same cause, namely, the wrinkling up of the chilled surface-crust by the movement of the liquid mass below. Lavas which present this appearance are frequently called 'ropy lavas'; an admirable example of them is afforded in the lava-cascade of the Island of Bourbon represented in fig. 18 (page 93).

But lavas in which the rate of flow has been very rapid, exhibit quite a different kind of surface to that of the ropy lavas. The Vesuvian lava-stream of 1872 was remarkable for the rapidity of its flow, and its surface presents a remarkable contrast to that of the slow-moving lava of 1858. The surface of the lava-current of 1872 is covered with rough cindery masses, often of enormous dimensions, and it is exceedingly difficult to traverse it, as the ragged projecting fragments tear the boots and lacerate the skin. The

FIG. 23.-VESUVIAN LAVA-STREAM OF 1872, EXHIBITING THE ROUGH CINDERY SURFACES CHARACTERISTIC OF