We have already referred briefly to the _lava caves_ or _grottoes_, that are formed in some of the lava streams issuing from Vesuvius, Etna, or Hawaii. These caves consist either of a number of communicating huge bubbles, or of the tunnels that are formed in the lava by the hardening of the outside of the lava streams as they flow down the sides of the mountain, and towards the close of the eruption are afterwards emptied by the molten lava within continuing to flow to a lower level before solidifying. Now, in the interior of these caves, there are often found on the walls, as well as on the portions of the floors of the caves, immediately below them, curious pendants, like icicles, or, more correctly, like the _stalactites of limestone_ that are seen hanging to the walls of caves in limestone districts, where they are formed as follows: as the rain water sinks through limestone strata it dissolves some of the lime, when, slowly falling, drop after drop, from the roofs of the caverns, small particles of lime are deposited on the roof, and in this manner a pendant of limestone is formed. The water that falls to the floor of the causeway immediately below, also builds up a dome-like hillock called a stalagmite. In due time the pillar reaches downwards, and the opposite hillock upwards until the two meet, thus forming great natural pillars that appear to hold up the roof of the vast cave in which they have been slowly formed. A number of _lava stalactites_ are represented in Fig. 34. [Illustration: FIG. 34. LAVA STALACTITES _From Dana's Manual of Geology_] Now, in a similar manner these lava stalactites, formed in the lava caves or grottoes, are caused by the stream as it escapes from the walls of the caves depositing on them stalactites of various lava minerals it has dissolved as it slowly passed through them. But the most important of all volcanic products is _volcanic dust_. This, as we have seen, is so light that it remains longest in the air, and is often carried by the winds to great distances from the volcano from which it escaped. It may interest you to know that some of the most fruitful of the great wheat fields of the western parts of the United States owe their extraordinary fertility to immense deposits of volcanic dust that have been thrown out from some of the great volcanoes of the geological past, now found in an extinct condition in these parts of the United States. According to Russell, immense deposits of volcanic dust are spread over vast areas in Montana, Southern Dakota, Nebraska, and Kansas, as well as over parts of Oregon, and Washington, and, indeed, over large areas of southwestern Canada and Alaska.