John Smith Flett

“ A few of the most important clay rocks, such as china-clay, brick-clay, red-clay and shale, may be briefly described here.
China-clay is white, friable and earthy. It occurs in regions of granite, porphyry and syenite, and usually occupies funnel-shaped cavities of no great superficial area, but of considerable depth. It consists of very fine scaly kaolin, larger, shining plates of white mica, grains of quartz and particles of semi-decomposed felspar, tourmaline, zircon and other minerals, which originally formed part of the granite. ”

“ Kaolin and muscovite are formed principally after felspar (and the felspars are the commonest minerals of all crystalline rocks) ; also from nepheline, leucite, scapolite and a variety of other rock-forming minerals. Chlorite arises from biotite, augite and hornblende. Serpentine, which may be fibrous or scaly, is a secondary product of olivine and certain pyroxenes. Clays consist essentially of the above ingredients (although serpentine is not known to take part in them to any extent, it is closely allied to chlorite) . At the same time other substances are produced as decomposition goes on. ”

“ They are always very impure.
The red-clay is an abysmal formation, occurring in the sea bottom in the deepest part of the oceans. It is estimated to cover over fifty millions of square miles, and is probably the most extensive deposit which is in course of accumulation at the present day. In addition to the reddish or brownish argillaceous matrix it contains fresh or decomposed crystals of volcanic minerals, such as felspar, augite, hornblende, olivine and pumiceous or palagonitic rocks. ”

“ Near volcanoes they contain many volcanic minerals, and around coral islands they are often in large part calcareous.
Microscopic sections of some of the more coherent clays and shales may be prepared by saturating them with Canada balsam by long boiling, and slicing the resultant mass in the same manner as one of the harder rocks. They show that clay rocks contain abundant very small grains of quartz (about 0·01 to 0·05 mm. in diameter) , with often felspar, tourmaline, zircon, epidote, rutile and more or less calcite. These may form more than one-third of an ordinary shale ”

“ It happens, however, that many rocks are not disintegrated to this extreme degree by natural processes, and weathering invariably accompanies disintegration. Quartz, for example, has little or no cleavage, and is not attacked by the atmosphere. ”

“ Some of these are recognizable as pale yellowish and white mica; others seem to be chlorite, the remainder is perhaps kaolin, but, owing to the minute size of the flakes, they yield very indistinct reactions to polarized light. They are also often stained with iron oxide and organic substances, and in consequence their true nature is almost impossible to determine. It is certain, however, that the finer-grained rocks are richest in alumina, and in combined water ”

“ The coarser materials settle nearer the land, and the shallower portions of the sea floor are strewn with gravel and sand, except in occasional depressions and near the mouths of rivers where mud may gather. ”

“ It is fine, tenacious and bright red, and represents the insoluble and thoroughly weathered impurities which are left behind when the calcareous matter is removed in solution by carbonated waters. Similar residual clays sometimes occur on the surface of areas of limestone in hollows and fissures formed by weathering. ”

“ These deposits are tenanted by numerous forms of marine life, and the sulphur they contain is derived from decomposing organic matter. Occasionally water-logged plant débris is mingled with the mud. In a few places a red colour prevails, the iron being mostly oxidized ”