Outlines of field-geology / by Professor Geikie.

  • Archibald Geikie
Date:
1876
Catalogue details

Licence: Public Domain Mark

Credit: Outlines of field-geology / by Professor Geikie. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    kat the surface. Their existence must be determined from mther evidence, therefore, than actual visible sections of them. If the geologist be at work in a district where veins of this dund occur, he should endeavour as early as possible to make lumself familiar with the characteristic mineral substance (which may constitute the chief part of the veins. Such .minerals as quartz, barytes, calcite, and other “ vein-stones,” iis they are called, are of common occurrence, but often exhibit local peculiarities by which they may be recognised and traced to their sources. Having examined the visible sections of some of the mineral veins, and learnt the way in fwhich the vein-stones are associated with any metalliferous core, he may be on the watch for evidence of the occurrence :of the veins elsewhere. He follows with this view the same cplan as that which I have already described with reference to the tracing of the limits of formations by means of scattered (surface fragments. In ascending a stream or a hill-side, he c takes note of any marked number of pieces of vein-stone, and :of the point beyond which they grow fewer or cease. Having thus got a rough indication of the existence of one tor more veins, he proceeds to a more minute search over that {part of the ground, and unless the rocks should be too much tconcealed, he may hope to meet with an indication of the tactual outcrop of the vein. It is not always, nor perhaps :often, safe to pronounce as to the commercial value of such ta vein from surface evidence of this kind. The rock may ineed to be opened up, and boring or mining carried for some Tway below the surface before a reliable opinion can be ex- pressed as to whether or not the vein may be worked to : profit. 6. Construction of Sections. The more clearly a geological map represents the structure of a country, the less need is there for any additional •explanation, so that a perfect map, large in scale and detailed in execution, should be independent of sections or other ! assistance,^ except for data, which cannot be expressed upon a map. But such a map can comparatively seldom be con- 1 stracted. W e must bo content with a small scale, an im- perfect topography, and other defects which compel us to