Elements of agricultural chemistry, in a course of lectures of the Board of Agriculture / By Sir Humphry Davy.

  • Davy, Humphry, Sir, 1778-1829.
Date:
1846
    Fifth, porphyry, which consists of crystals of felspar, embedded in the same material, but usually of a different colour. Sixth, granular marble, which consists entirely of crystals of carbonate of lime; and which, when its colour is white, and texture fine, is the substance used by statuaries. Seventh, chlorite schist, which consists of chlorite, a green or gray sub- stance, somewhat analogous to mica and felspar. Eighth, quartzose rock, which is composed of quartz in a granular form, sometimes united to small quantities of the crystalline elements, which have been mentioned as belonging to the other rocks. The secondary rocks are more numerous than the primary : but twelve varieties include all that are usually found in these islands. First, grauwacke, which consists of fragments of quartz, or chlorite schist, embedded in a cement, principally composed of felspar. Second, siliceous sandstone, which is composed of fine quartz or sand, united by a siliceous cement. Third, limestone, consisting of carbonate of lime, more compact in its texture than the granular marble; and often abounding in marine exuvia. Fourth, aluminous schist, or shale, consisting of the decomposed materials of different rocks cemented by a small quantity of ferruginous or siliceous matter ; and often containing the impressions of vegetables. Fifth, calcareous sandstone, which is calcareous sand, cemented by cal- careous matter. Sixth, ironstone, formed of nearly the same materials as aluminous schist, or shale; but containing a much larger quantity of oxide of iron Seventh, basalt or whinstone, which consists of felspar and hornblende, with materials derived from the decomposition of the primary rocks; the crystals are generally so small as to give the rock a homogeneous appear- ance ; and it is often disposed in very regular columns, having usually five or six sides. Eighth, bituminous or common coal. Ninth, gypsum, the substance so well known by that name, which con- sists of sulphate of lime ; and often contains sand. Tenth, rock salt. Eleventh, chalk, which usually abounds in remains of marine animals, and contains horizontal layers of flints. Twelfth, plum-pudding stone, consisting of pebbles cemented by a fer- ruginous or siliceous cement. To describe more particularly the constituent parts of the different rocks and strata will be unnecessary; at any time, indeed, details on this subject are useless, unless the specimens are examined by the eye; and a close inspection and comparison of the different species, will, in a short time, enable the most common observer to distinguish them. The highest mountains in these islands, and indeed in the whole of the old continent, are constituted by granite ; and this rock has likewise been found at the greatest depths to which the industry of man has as yet been able to penetrate; micaceous schist is often found immediately upon granite; serpentine or marble upon micaceous schist: but the order in which the primary rocks are grouped together is various. Marble and serpentine are usually found uppermost; but granite, though it seems to form the foundation of the rocky strata of the globe, is yet sometimes discovered above micaceous schist. The secondary rocks are always incumbent on the primary; the lowest of them is usually grauwacke: upon this, limestone or sandstone is often
    found; coal generally occurs between sandstone or shale; basalt often exists above sandstone and limestone; rock salt almost always occurs as- sociated with red sandstone and gypsum. Coal, basalt, sandstone, and limestone, are often arranged in different alternate layers, of no consider- able thickness, so as to form a great extent of country. In a depth of less than 500 yards, 80 of these different alternate strata have been counted. The veins which afford metallic substances, are fissures more or less vertical, filled with a material different from the rock in which they exist. This material is almost always crystalline; and usually consists of cal- careous spar, fluor spar, quartz, or heavy spar, either separate or together. The metallic substances are generally dispersed through, or confusedly mixed with these crystalline bodies. The veins in hard granite seldom afford much useful metal; but in the veins in soft granite and in gneiss, tin, copper, and lead are found. Copper and iron are the only metals usually found in the veins in serpentine. Micaceous schist, syenite, and granular marble, are seldom metalliferous rocks. Lead, tin, copper, iron, and many other metals, are found in the veins in chlorite schist. Grau- wacke, when it contains few fragments, and exists in large masses, is often a metalliferous rock. The precious metals, likewise iron, lead, and anti- mony, are found in it; and sometimes it contains veins or masses of stone- coal, or coal free from bitumen. Limestone is the great metalliferous rock of the secondary family ; and lead and copper are the metals most usually found in it. No metallic veins have ever been found in shale, chalk, or calcareous sandstone ; and they are very rare in basalt and siliceous sandstone. In cases where veins in rocks are exposed to the atmosphere, indications of the metals they contain may be often gained from their superficial appear- ance. Whenever fluor spar is found in a vein, there is always strong reason to suspect that it is associated with metallic substances: A brown powder at the surface of a vein always indicates iron, and often tin ; a pale yellow powder, lead; and a green colour in a vein denotes the presence of copper. It may not be improper to give a general description of the geological constitution of Great Britain and Ireland. Granite forms the great ridge of hills extending from Land’s end through Dartmoor into Devonshire. The highest rocky strata in Somersetshire are grauwacke and limestone. The Malvern hills are composed of granite, syenite, and porphyry. The highest mountains in Wales are chlorite-schist, or grauwacke. Granite occurs at mount Sorrel in Leicestershire. The great range of the moun- tains in Cumberland and Westmoreland, are porphyry, chlorite, schist, and grauwacke ; but granite is found at their western boundary. Through- out Scotland the most elevated rocks are granite, syenite, and micaceous schistus. No true secondary formations are found in South Britain, west of Dartmoor; and no basalt south of the Severn. The chalk district ex- tends from the western part of Dorsetshire, to the eastern coast of Norfolk. The coal formations abound in the district between Glamorganshire and Derbyshire ; and likewise in the secondary strata of Yorkshire, Durham, Westmoreland, and Northumberland. Serpentine is found near Cape Lizard in Cornwall, at Portsoy in Banffshire, and in Ayrshire. Black and grey granular marble is found near Padstow in Cornwall; and other coloured primary marbles exist in the neighbourhood of Plymouth. Coloured primary marbles are abundant in Scotland; and white granular marble is found in the Isle of Skye, in Assynt, and on the banks of Loch Shin in Sutherland : the principal coal formations in Scotland are in Q
    Dumbartonshire, Ayrshire, Fifeshire, and on the banks of the Brora in Sutherland. Secondary limestone and sandstone are found in most of the low countries north of the Mendip hills. In Ireland there are five great associations of primary mountains; the mountains of Morne in the county of Down; the mountains of Donegal; those of Mayo and Galway, those of Wicklow, and those of Kerry. The rocks composing the first four of these mountain chains are principally granite, gneiss, syenite, micaceous schist, and porphyry. The mountains of Kerry are chiefly constituted by granular quartz, and chlorite-schist. Coloured marble is found near Killarney ; and white marble on the western coast of Donegal. Limestone and sandstone are the common secondary rocks found south of Dublin.* In Sligo, Roscommon, and Leitrim, limestone, sandstone, shale, iron stone, and bituminous coal are found. The secondary hills in these counties are of considerable elevation; and many of them have basaltic summits. The northern coast of Ireland is principally basalt; this rock commonly reposes upon a white limestone containing layers of flint, and the same fossils as chalk ; but it is considerably harder than that rock. There are some instances, in this district, in which columnar basalt is found above sandstone and shale, alternating with coal. The stone- coal of Ireland is principally found in Kilkenny, associated with limestone and grauwacke. * It is evident from what has been said concerning the production of soils horn rocks, that there must be at least as many varieties of soils as there are species of rocks exposed at the surface of the earth ; in fact, there are many more. Independent of the changes produced by cultivation and the exertions of human labour, the materials of strata have been mixed together, and transported from place to place by various great alterations that have taken place in the system of our globe, and by the constant operation of water. To attempt to class soils with scientific accuracy, wpuld be a vain labour ; the distinctions adopted by farmers are sufficient for the purposes of agriculture ; particularly if some degree of precision be adopted in the application of terms. The term sandy, for instance, should never be applied to any soil that does not contain at least J of sand; sandy soils that effervesce with acids should be distinguished by the name of calcare- ous sandy soils, to distinguish them from those that are siliceous. The term clayey soil should not be applied to any land which contains less than one-sixth of impalpable earthy matter, not considerably effervescing * Fig 25 will give a general idea of the appearance and arrangement of rocks and veins:— 1. Granite. 2. Gneiss. 3. Micaceous Schistus. 4. Syenite. 5. Serpentine. 6 Porphyry. 7. Granular Marble. 8. Chlorite Schist. 9. Quartzose Rock. 10. Grauwacke. 11. Siliceous Sandstone. 12. Limestone. 13. Shale. 14. Calcareous Sandstone. 15. Iron Stone. 16. Basalt. 17. Coal. 18. Gypsum. 19. Rocksalt. 20. Chalk. 21. Plum-pudding Stone. aa. Primary Mountains. bb. Secondary Mountains. aa. Veins.—D.
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