While lava is cooling, there is a constant escape of gases, mostly steam, and as these rise through the molten mass they make cavities, near the upper surface, that portion on top often becoming frothy. If this solidifies quickly so that the gas cavities are preserved it is scoria. When the gas cavities are small and uniformly distributed, the rock is called pumice, and often used as a scouring agent. When the cavities are large and irregular the term scoria is generally used. Molten lavas may form various structures, according to the conditions under which they cool, dripping through cracks or from the roof of caves, which often form where the molten lava escapes from a hardened shell, and making stalactites, stalagmites, etc. The very thin lava of the Hawaiian volcanoes may even be blown by the wind into fine threads, known as “Pele’s hair.” The presence of the gas cavities is so characteristic of the upper surface of lavas which have been extruded; that, where one is dealing with older lavas, now buried beneath other rocks, this fact helps to determine whether the mass is a sheet, rather than a sill; for, in the case of the sill, the lava was forced between layers of sedimentary rocks, and the burden of the overlying rocks did not permit the escape of steam and therefore the upper surface of sills does not have the scoriaceous structure. Amygdoloid Pl. 56 When the upper surface of a lava is filled with steam holes, and this lava has been buried beneath other rocks, the seeping waters slowly bring such minerals as quartz, calcite and zeolites and fill the cavities. Such a rock is known as an amygdoloid. It is often confused with porphyry; but, if examined closely, it will be seen that the outlines of the gas cavities are rounded, while the outlines of a crystal, like a phenocryst, are always angular. This will be clear if the amygdoloid on Plate 56 is compared with the porphyry on Plate 55. The Sedimentary Rocks To this class belong all those rocks which have been laid down by water or wind, or are the results of organic depositions. They include loose material like sand or day, and also the same materials, when cemented into more or less solid rocks, like sandstone or shale. So long as the material has not been altered from what it was when laid down, the rock is termed sedimentary.