All rocks present in gold deposits show lines of fracture of different structure. Some lines present a series of parallel planes along which they split with great facility, sometimes along the lines of original gold deposits. Stratification is the result of early matter being deposited in water as layer after, with intervals of time between the deposition of the layers, during which the first layer deposited had time to harden or form a sort of crust which prevented it mixing freely with the succeeding layer and so on; so that the mass has become like a series of sheets of paper laid one upon the other, and when converted into rock by the lapse of time, and raised out of water, the rocks split easily along these lines of deposit.
The layers may vary greatly in thickness, depending on the amount of sediment brought down by the stream and the length of the flood periods. It is easy to understand that a stream during flood will carry immense quantities of matter into a lake or the ocean, becoming clear during periods of drought, thus fulfilling the conditions called for; and that such streams as the Mississippi, Amazon and Ganges may form beds of vast extent, while others may be limited to the area of a small lake. It is important to mention that rocks formed in this manner are known as stratified.
In the case of Cleavage, certain rocks such as roofing slates, while belonging to the stratified series, have become so altered by pressure that they no longer split along the lines of stratification or deposit, sometimes known as lamination lines, but on a series of joints which have been subsequently formed by this pressure, generally more or less at right angles to the lines of the original deposit. The old lines of lamination are obliterated and this new series of joints is much more numerous than the original horizontal planes and the splitting character much more perfect, dividing the rock into very thin sheets. This structure is known as cleavage. In crystal s of minerals, the cleavage is the line on which the mineral splits most readily, and is usually parallel to one of the smooth faces of the crystals.
In addition to these splitting planes, all rocks have acquired two or three sets of joints, more or less at right angles to each other, which divide the mass into large blocks and greatly facilitate the labor of gold miner. These may be altogether independent of stratification, being the results of the upheaval and compression of the earth’s crust, though in some cases they may follow some of the more or less horizontal lines. Upon their character frequently depends on the shape of gold bearing mineral veins and deposits. The joints which are roughly horizontal or parallel to the original stratification are called bedding planes or dip joints and the series running with the general trend of the rocks through the country, the strike joints. These joints may be very numerous or wide apart, and are the cleanest cut in close fine grained rocks. These joints may be only a few yards long, or may extend for a mile, the latter feature being most prominent in close grained rocks which have suffered comparatively little disturbance.
All rocks are constantly changing their character. The mud banks of today will be the shales and slates of the far future and the sand banks, the sandstones of a coming era. In the same way and by the same agencies of time, moisture, pressure and heat, many of the older sedimentary rocks have lost much of their original character. The sandstones have become massive quartzites, in which small grains of quartz are not detected easily. And the slates and shales have lost many of their lines of stratification, besides suffering other changes which have imparted to them a character known as schistose. Such rocks are called schists.