Perlite is a glassy lava, containing two to four per cent of water, which, on cooling, has cracked into numerous rounded masses, with a concentric structure, reminding one of the layers of an onion.
[Scoria]
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.
In general the material of which these rocks are composed comes from the weathering and disintegration of other rocks. This does not apply to the organic deposits, for each type of which there is a peculiar mode of formation. To illustrate the typical formation of sedimentary rocks, we may look at the fate of a granite when exposed. At once the surface is attacked by changes of temperature, frost and rain. The various minerals of the granite expand and contract with every change of temperature, but each component mineral has a different coefficient of expansion under heat, so that minute cracks are quickly formed between the minerals. Water gets into these cracks and begins to dissolve the minerals. Feldspar is the most easily attacked, part of it being dissolved and carried away, a small part changing to quartz, and by far the largest part changing to kaolin. The dark mineral is also attacked and partly dissolved, and partly changed to kaolin and iron oxides. The quartz resists solution almost completely. Of these products the kaolin and iron oxides are carried far away and deposited in still water. The quartz and perhaps some of the dark mineral are heavier and carried more slowly, being deposited as sand. This happens to granite everywhere, but in the regions where there is frost the action is greatly hastened; for water gets into the cracks and expands every time it freezes and thus widens the cracks rapidly, which greatly facilitates the entrance and movement of water in the rock. In a similar way any original rock will be disintegrated, and the residue, after the soluble part has been carried away, becomes sand or clay or mud.
Particles of quartz, kaolin, and lime, separately, or mixed, loose or more or less cemented, with accompanying impurities, make up the great bulk of the sedimentary rocks. They are usually arranged in layers, of varying thickness, as they were laid down by water or the wind. In the same way layered accumulations which are either products of plants or animals, or parts of the plants or animals, are considered sedimentary, as for instance, coal, chalk, petroleum, etc.