CO885-(21-23) — Page 482

PUBLIC RECORD OFFICE

Reference :-

PERE CO. 885

22 PUBLIC RECORD OFFICE, LONDON

ALLY WITHOUT PERMISSION OF THE BE REPRODUCED PHOTOGRAPHIC- COPYRIGHT PHOTOGRAPH-NOT TO

6

as a rule from that of the cleavage, while the strike is usually, but by no means always, the same.

The outcrop of a stratum is that portion of the surface which it occupies. When the surface is horizontal or the beds are vertical the outcrop forms a strip parallel to the strike. When the beds are horizontal, the outcrop follows the contours. other cases it depends partly on the dip and partly on the form of the surface.

In

When the strata are curved and the convex side of an undulation is directed upwards, they are described as forming ** anticline, and when downwards, a syncline.' The collector however need not, as such, concern himself with these structures, unless they are obviously present in the rocks of a quarry or cliff from which a specimen is taken.

Cases occur, where the rocks are found to be broken across, and one side of the fracture has been moved relatively to the other, constituting what is known to geologists and miners as a fault." If the plane of a fault fracture is not vertical, it is said to hade," and we speak of the direction of hade in the same manner as of the direction of dip, but the angle of hade is measured from the vertical, not from the horizontal as in the case of the angle of dip. The term strike is used with reference to a fault plane in the same way as is with reference to a bedding plane. Thus a nearly vertical fault may hade at an angle of 50 to the east and have a north and south strike. Miners sometimes use the term "underlie" or "underlay ", in the sense of hade. The movement at a fault may take place directly up or down the fault plane, horizontally along the strike, or in an intermediate direction. The amount of relative vertical movement which appears to have taken place between the two sides of a fault is known as the "throw."

V.

ECONOMIC PRODUCTS.

Economic mineral products sometimes occur in beds or seanis in sedimentary rocks or crystalline schists, as is the case with coal and some of the iron ores of Nigeria, and sometimes as '' lodes," "veius" or "pockets." Lodes are fissures wholly or partially filled with mineral substances brought by water or vapour. Among these are the ores of tin, copper, lead, zine and other metals. Lodes are usually steeply inclined and are frequently planes of faulting. In describing a lode the terms hade and strike are used in the same manner as when applied to a fault. A vein is a lode of irregular form, and it frequently branches. A pocket is, as its name implies, of limited extent both horizon- tally and vertically.

VI.

DECOMPOSITION AND WEATHERING.

If the surface of a rock has been long exposed to the action of the air or water it becomes more or less decomposed and is said to be weathered. In climates which are sufficiently moist for vege- tation to flourish there is usually a dark layer of decomposed

7

rock and carbonaceous material which forms the "soil" and extends to a variable depth. If it has no definite lower boundary

it is usually considered to extend to a depth of nine inches. The decomposed rock below is known as the subsoil.

In warm moist climates decomposition proceeds rapidly and most rocks except those which are highly siliceous are converted to a considerable depth into clay or claylike material. For this reason, as well as on account of the dense growth of the vegeta- tion, it unusual in such a climate to find exposures of unaltered rocks, except in the courses of rapid streams, which remove all decomposed rocks.

In dry climates on the other hand the rock is often found at the surface in a comparatively unaltered state and the absence of a thick vegetable covering renders the work of collecting speci- mens comparatively easy.

The loose incoherent material resulting from the decomposition of rocks is carried away by the rain and wind and usually finds its way sooner or later to the nearest stream or river where it is sorted by the action of the running water-the lighter and finer material being as a rule carried furthest, while the heavier and courser constituents are laid down close at hand. In this way heavy minerals such as tinstone and gold which may have been originally scattered in small grains through great masses of rock are collected in the banks of sand and gravel in a river in much larger proportion than in the original rock and occasionally the deposits may be rich enough to repay the cost of working. Such fluviatile beds containing valuable minerals are known as alluvial

placer" deposits.

or

VII.

WHAT TO COLLECT.

The collector should endeavour so far as possible to obtain specimens of all the different types of rock that are met with in the area which is the scene of his operatious, as well as of the minerals and fossils associated with them.

In the case of intrusive rocks any variations of texture or composition should be represented in the collection. In particular, specimens should be obtained not only from the centre of dykes or sills but from their margins, where they are in contact with other rocks. Specimens from the margins of massive intrusions are equally important.

The rock adjoining an intrusive rock is frequently altered or metamorphosed by the heat of the latter and the vapours it gave out when in a molten state, and specimens of such metamor- phosed rocks should likewise be secured.

Where the surface of the rock is weathered or decomposed, every effort should be made to obtain specimens of the fresh unchanged material. The decomposed rock is, however, fre quently of scientific and economic interest especially in connec tion with agricultural questions and should not be neglected if time permits of specimens being taken of it.

If samples of mineral products are required to determine their commercial value, they should be taken from as great a depth