Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/459

Rh MINING 441 Fi?. 1. inclination downwards measured from the horizontal ; its strike is the direction of a horizontal line drawn in the middle plane. The thickness of beds that are worked varies within very wide limits. Whilst the thickness of certain workable beds of coal is only 1 foot, and that of the Mansfeld cupriferous shale only 10 to 20 inches, we find on the other hand one of the beds of lead-bearing sandstone at Mechernich no less than 85 feet thick, and beds of slate far exceeding that thickness. It must not be supposed, however, that the thickness of a bed necessarily remains uniform. Occasionally this is the case over a very large area ; but frequently the thickness varies, and the bed may dwindle away gradually, or in crease in size, or become divided into two owing to the appearance of a parting of valueless rock. Fig. 1 shows beds of shale, lime stone, iron ore, and sandstone. Any one of these beds may be valuable enough to be worked. (2) Mineral Veins or Lodes. Veins or lodes are tabular or sheet-like deposits of mineral which have been formed since the rocks by which they are surrounded ; they differ, therefore, by their subsequent origin from beds, which, as just stated, are of contemporaneous origin with the enclos ing rocks (although of course cases occur in which the deposit is lying unconformably upon very much older strata, or is covered unconformably by very much younger strata). It is necessary to explain that the term &quot; vein &quot; in this definition is used in a more restricted sense than is sometimes customary among miners, who speak of veins of coal, clay-ironstone, and slate, which geologically are true beds. They see a band of valuable mineral or rock, and, careless of its origin, call it metaphorically a vein or seam. On the other hand, the definition is broader than that which prevails among some geologists, who would confine the term vein to deposits occupying spaces formed by fissures. The term &quot;lode&quot; was denned in 1877 by Mr Justice Field in the celebrated Eureka v. Richmond case as follows: &quot;We are of opinion, therefore, that the term, as used in the Acts of Congress, is applic able to any zone or belt of mineralized rock lying within bound aries clearly separating it from the neighbouring rocks.&quot; This interpretation seems suitable for the peculiar mining tenure of the United States, where the discoverer of a vein or lode can obtain a mining claim of 500 yards in length along the lode. It protects the prospector, whose object is to obtain a secure title, the mode of origin of the deposit being a matterof small importance to him so long as it is worth working. In many cases also it would be impossible to decide upon the mode of origin until workings had progressed considerably, and even then there would be room for disputes. No doubt a very large number of mineral veins are simply the contents of fissures ; others are bands of rock impregnated with ore adjacent to fissures or planes of separation ; others, again, have been formed by the more or less complete replacement of the constituents of the original rock by particles of ore. Veins may occur in igneous or in sedimentary rocks, and in the latter they frequently cut across the planes of stratification. Like a bed, a vein has its dip and strike ; but, as the dip of veins is generally great, the inclination is usually measured from the vertical, and is then spoken of as the underlie or hade. The bounding planes of a vein are called the walls or checks, and they are frequently smooth and striated, showing that one side must have slid against the other. The upper wall is known as the lutnying wall, the lower one as the foot ivall. The width of a vein is measured at right angles to the walls. A typical example of a fissure-vein is shown in fig. 2, repre senting a lead lode in slate at Wheal Mary Ann mine 1 in Cornwall. 1 C. Le Neve Foster, &quot; Remarks on the Lode at Wheal Mary Ann, Menheniot,&quot; Trans. Roy. Geol. Soc. Cornwall, vol. ix. p. 153. Fig. 3. It is evident that a fissure in the surrounding slate has here been filled up by the successive deposition of bands of mineral on both sides. A large proportion of the contents of a lode may consist of fragments of the walls that have fallen into the original fissure, and these are often tightly cemented ^^ together by minerals that have ^ ) been introduced subsequently. The ^ horizontal section of part of the ^ Comstock lode -* (Plate IV.) shows 3 much &quot;country&quot; rock enclosed =i~=i=5 within the walls. Where a lode consists of rock impregnated with ore, the mineral ized part may fade away gradually into the surrounding rock (country) without there being any distinct Fig. 2. wall, as shown in fig. 3, which is an illustration taken from the Great Flat Lode 3 near Redruth in Cornwall. The celebrated Ruby Hill deposit in the Eureka district, Nevada, is a mineralized zone of dolo- initic limestone varying in . width from a few inches to ***&amp;lt; 450 feet, and having a mean width of 250 feet. It con tains numerous irregular ore- bodies, which consist mainly of highly ferruginous car bonate of lead, rich in silver and gold. This mineralized limestone band, long called a lode by miners, has been determined by the decision just men tioned to be a lode in the eyes of the law. Veins often continue for a great distance along their strike. The Van lode in Montgomeryshire is known for a length of 9 miles, whilst the Great Quartz Vein in California has been traced for a distance of no less than 80 miles. Veins are of less uniform pro ductiveness than beds, and are rarely worth working throughout. Rich portions alternate with poor or worthless portions. The rich parts have received various names according to the forms they assume : fig. 4 represents a Su p&amp;lt;= L, N longitudinal section along the &quot;*&quot; i ! / r 11 i ADIT LEVEL ^Vo?^fe i .s^n^ strike (course) 01 a lode, and ^......... TJ.r .ftis c ..ft jflgg? the stippled parts are ore- &quot;&quot;&quot; TH &quot; /? ^: i ; : :v^^ .&amp;gt;* &quot;/&/&quot; &quot; bodies; B, B, Bare bunches; A =li=^;;!V&amp;gt;^-;... ^-:V:..--- 7t ^^ L ...frffl is a large bunch or course of .. 3 ff.: ;. . . .-, - .....^ .f f^ %r- -1 == ore ; when an ore-body forms J^^!;i :. : ^l : :!: ^ &amp;lt;; Z$^ ~ a sort of continuous column so&quot; &quot;- --- &quot;&quot; &quot;&quot; $ ^_ we have a shoot, and ore-. LTVL c x .. V^JB bodies which on being ex- pj K 4 cavated leave chimney-like openings are called trijxs (fig. 4, C). In the United ^States the Spanish word bonanza, literally meaning &quot;fair weather&quot; or &quot;pro sperity,&quot; is frequently used for a rich body of ore. The richness of veins is dependent in many cases upon the nature of the adjacent rock (country), upon the underlie, and upon the strike, variations in any one of these three elements being often sufficient to cause a decided change of productiveness. Various theories have been formed concerning the origin of mineral veins. Some geologists suppose that the minerals now constituting the veins have been dissolved out of the adjacent rocks and re-deposited in the vein cavity ; others, on the contrary, believe that the ores have been brought up from great depths by mineral springs. In all probability both theories are correct, some lodes having been formed by the former process and some by the latter ; and, furthermore, other lodes appear to owe their origin to a gradual substitution of valuable minerals in the place of some of the con stituents of a worthless rock. One of the most important con tributions to the science of ore-deposits of late years has been the discovery by Professor F. Sandberger of small quantities of silver, lead, copper, nickel, cobalt, bismuth, arsenic, antimony, and tin in silicates, such as olivine, augite, hornblende, and mica, which are constituents of igneous rocks. He therefore regards these rocks as the sources from which lodes have derived their riches. B. Masses. These are deposits of mineral, often of Masses, irregular shapes, which cannot be distinctly recognized as beds or veins. Such, for instance, are the red hematite 2 James D. Hague, in United States Geological Exploration of the Fortieth Parallel, vol. iii., &quot;Mining Industry,&quot; Washington, 1870, Atlas, plate 11. 3 C. Le Neve Foster, &quot;On the Great Flat Lode south of Ked- ruth and Camborne, and on some other Tin Deposits formed by the Alteration of Granite,&quot; Quart. Jour. Geol. Soc., vol. xxxiv. p. 644. XVI. 56