Correspondence.

Nelson Examiner and New Zealand Chronicle, Volume XVIII, Issue 94, 23 November 1859, Page 3

Correspondence.

Sib— Some time ago you inserted an article on "Seeking for Coal," from a work by Colonel Jackson. As this excited some attention at the time, especially in Auckland, where Dr. Hochstettev was then exploring ; and as Dr. Hochstetter has since confirmed the views of many previous writers on the geology and mineral resources of this province ; and as the indications of coal pointed out in that article so exactly coincide with those at Mr. Jenkins's mine ; the following extract from the same work on the different qualities of coal proper may not be without interest, and may be of some service to those engaged in the important work of raising this invaluable combustible from the bowels of the earth. I am, &c. E. TUOKEE. COAL. GENERAL CHARACTERS. Coal is almost a perfect black colour and generally shining ; it is not hard, it is generally brittle, but : never soft enough to be scratched with the nail) its 1 mean specific gravity is I*3. It burns easily, with a white flame, and black emote, giving out a particular bituminous and in | Boms cases sulphurous smell ; it leaves after burning a cinder, sometimes very voluminous,bemg about three per cent., but it sometimes leaves nothing but a white ash in small quantity. Coal cinders are hardly ever pulverulent, they are generally in the form of a light scoria, or of a dust mixed with scorife. This combustible by distillation gives out au empy« reumatic oil, ammoniacal gas, and sometimes sulphurous acid gas without ammonia. Coal has never been found crystallized ; it is always in mass, having sometimes a slafcy texture, and often a conchoidal, though more generally a straight fracture ; in the latter case the mass generally divides in parallelopipedons of considerable regularity ; finally, in some varieties the surface of the fragments exhibits the most beautiful and varied colours.

To the Editor of the ' Nelson Examinee.'

Such are the chief characteristics common to the ] different varieties of coal, and which serve to distinguish it from all other combustibles ; the facility with which it burns and the smoke it gives out distinguish it from Anthracite; its solidity prevents its being confounded with AsphaUuai, which may be cut with the nail, and which gives out, when rubbed between the fingers, a very sonsible bituminous odour ; finally Lignite, which very much resembles coal, gives out by distillation an acid and empyreumatic liquor. The varieties of coal differ so slightly from each, other, and their points of resemblance are so numerous, that it is very difficult to determine them with precision. However as these varieties have different uses in the arts, it is desirable to know, at least, the principal of them. Werner establishes six different varieties. We, however, prefer the classification of M. Brogniart, who recognises only three well-distinguished varieties : —

" Compact Coal. — This variety is of a grayish black and without lustre ; its fracture is sometimes largely conchoidal, aud sometimes straight with an even surface ; it is solid without being hard, and though compact is very light. Its specific gravity according to Kirwan, is 123 j it ii easily cut and takes » good polish.

It burns well, with a brilliant flame, gives out but little heat, and leaves hardly any residuum. Thi Icoal very much resembles Jet, only it does not giv« out in burning the same penetrating and disagreeablt smell that proceeds from Jet. Vases and other ornaments are made- of tin's kind of coal.

" Fat Coal. — This is light, brittle, very combustible, uid burns with a long white flame. It swells and seems to enter into a kind of fusion, while it runs together or cukes. It leaves little residue and yield? by distillation, bitumen, aud ammonia. Thi3 kind is found along with elate, but has never been seen in calcareous rocks. It sometimes contains marine fossils.

"Dry Coal.~- This kind of coal is both heavier and more compact than the first kind ; it is sometimes less intensely black, approaching to irongray ; it burns with difficulty, and without swelling or caking, and leaves a greater residue. There is scarcely any flame from it, and what there is, is bluish. By distillation it gives out neither ammonia nor bitumen, but sulphurous acid only. All the coal found along with compact limestone belongs to this variety. The slates which cover this variety contain rather impressions of ferns than of grasses." Geognostic Character. Coal is always found in mass, sometimes in heaps, but generally in strata, and very rarely in veins. The strata are variously inclined, dipping slightly, and being contorted in every possible way. Coal is almost everywhere accompanied by the same kind of rocks, the knowledge of which is essential, whenever we would seek for this valuable mineral, in order to avoid attempts often useless and always expensive. Coal is never found in primitive rocks nor among those of the more recent formations, as coarse limestone, chalk, Band, white and homogeneous sandstone, &c. Some combustible minerals are found in these rocks, but they must not be confounded with coal.

First Formation.— The coalfields, as they are called, vary little. Among the rocks of which they are composed we generally find in the following order : - 1, Micaoious and ferruginous sandstone, often very coarse grained (these sandstones are not exclusively composed of quar I z and mica, but contain fragments of stone of all kinds, particularly felspar; 2, Clayslates and mica-slates with impressions of fish and of vegetables belonging generally to the ferns and grasses ; 3, strata of marl, of limestone, or of indurated clay ; 4, a kind of secondary argillaceous rock, containing petrified branches, roots, and even whole trees ; 5, argillaceous iron-ore; 6, pebbles in a ferruginous sand. The coal of this formation is in strata of the rocks just mentioned, and which are indistinctly abovo and below the coal strata or seams ; it appears, however, that the coal is never found in the bed of pebbles, and that in general the contiguous rock both above aud below the coal is slate ; but that, below the coal often differs from that which is above it, and, what appears singular, it has been observed that the overlying slate is impregnated with bitumen, while the underlying contains none. Such, then, is the way in which coal, at least the best qualities of this combustible, are found : it nevertheless exists in two other kinds of formation.

Second Formation. — This is a series of trap-rocks in which the coal is found in very thick and extensive beds. The varieties usually met with in this repository are pitch-coal, moor-coal, coal-blend or " blind " coal, and sometimes slate-coal.

Third Formation. — This consists of compact limestone in thick and almost horizontal strata, in which are found powerful beds of coal. This limestone often contains fossil shells, and although the limestone be black by reason of the bitumen with which it is penetrated in the immediate vicinity of the coal, the shells retain a brilliant whiteness, which by the contrast produces a very agreeable effect. Coal has been met with in veins of eight yards thick in sandstone.

The repositories of coal are generally found midway between primitive and secondary mountains. Coal mines (?) are rarely found in the centre of the primitive mountains themselves ; when they are, it may be remarked that they rest against the slopes of these mountains at great heights. Coal mines are also rare in plains at a great distanco from mountain chains ; and when they do exist in such places, the coal is situated at a great depth, always in the slaterocks and below the limestone.

The strata of coal generally follow the direction of the valley on the flanks of which they are situated. Coal sometimes contains metals ; thus oxide of copper, sulphurel of mercury, native silver, gold, sulphuret of lead or galena, and antimony, have been found in it. Iron-ore is a frequent and most valuable accompaniment of coal. The coal of which we have spoken hitherto is the Black Coal, or coal properly bo called. There are, however, varieties of what is termed Brown Coal and Lignite which occur in alluvial formations, and are comparatively superficial. The brown coal is generally accompanied by clay and gravel ; the coal-beds do not preserve an equal thickness throughout their whole extent. The coal-seams of alluvial land are not interrupted by slips and dykes as is the case with the coal strata in other soils.