KNOAVLEDGE ♦ 



[Jan. 1, 1886. 



wriukled, and cracked, and crumpled features whicli other 

 agencies -would score more deeply in tlie face of the 

 globe, but which bestowed thereby a beaiity and variety 

 in valley, mountain, and table-land that no even surface 

 could boast. 



Our knowledge of this crust extends to only a rela- 

 tively small depth, the aggregate thickness of the strata 

 or layers of rock already measured being about twenty- 

 tive miles, or the one hiindred and sixtieth part of the 

 earth's semi-diameter. The term " rock " is applied 

 alike to hard granite and loose sand, to ore veined -with 

 metal, and to mud from country lanes, as including the 

 materiiLs comprising the crust or shell. Rocks are 

 divided into two classes — unstratified and stratified. 

 The vnsfrafiped, which are also called igneous or 

 Plutonic, embrace all rocks which, as the}- now 

 exist, have been fused together by heat, or erupted 

 from the earth's interior by volcanic agency. The 

 stratified, which are also called aqueous or Neptiinic, 

 embrace all rocks which have been deposited as sediment 

 by the action of water or air, or which are due to the 

 growth and decay of plants and animals. "With this class 

 are grouped the tnetamorphic, stratified rocks which have 

 been metamorphosed into a crystalline state by the action 

 of heat and pressure, resulting in effacement of their ori- 

 ginal character. Throughout the entire series of rocks 

 the newer have been, and are being, formed out of the 

 older, which, unless upheaved, are always found at the 

 bottom ; but of the original crust not a trace remains. 



The depths to which the unstratified rocks extend is 

 unknown, and as they contain no organic remains what- 

 ever, they tell us nothing concerning the origin and suc- 

 cessions of life on the earth. Their place in the con- 

 tinuity of geological history will appear as we advance ; 

 meanwhile we may pass to brief glance at that section of 

 the calendar which deals witli the stratified groups. For 

 convenience sake, and not as implying any gaps between 

 their several formations, save where natural causes have 

 operated, they are divided into five epochs. These, 

 together with the typical remains of plant and animal 

 associated with each, are shown in the following 

 table : — 



_ , Thichnfss 



-^J"""''- of strata. Plant. Animal. 



ArcheolithicorEozoic(i.('.,^/rt«7j- 



//T*"), chiefly Metamorphic ... 70,000ft. Algx* Monera.* 

 Primary or Palaeozoic {ancient 



J'.ft^) 42,000 Ferns Fishes. 



Secondary or Mesozoic (^middle 



fife) 15,000 Pine-forests Reptiles. 



Tertiary or Cainozoic (re cra< life) 3,000 (Leaf-bearing) ^j.^^ q,^], 



<. forests i 

 Quaternary or Post Tertiary E.xisting siiccies. 



COAL 



By W. Mattietj "Williams, 

 cannel coal and boghead. 



pact and 



of this is that 



soiling the fingers. 



ANNEL coal or "candle coal," so named 

 because some varieties are so highly bitu- 

 minous that a tapered piece may be 

 lighted at its angle and will continue for 

 awhile to burn like a candle, differs 

 mechanically from ordinary bituminous 

 coal in being harder and far more com- 

 homogeneous in structure. One result 

 it may be freely handled without 

 Some varieties of cannel coal re- 



Both marine. 



semble anthracite at first sight and handling, but an 

 expert can easily distinguish them. Anthracite is more 

 histrous than cannel. Whitby jet is a sort of cannel. 



One peculiar character of the cannels that are the 

 richest in flaming constituents is the " streak." 

 Although the coal itself is black, whether the fracture 

 be new or old, if it be scratched with the point of a knife 

 the streak thus made is brown. The value of a sample 

 for gas-making may be roughly tested thus — the lighter 

 the streak the richer the coal. 



Some cannels have a fracture which is quite smooth 

 and uniform, and slightly concave or conchoidal ; others 

 are curiously different. They appear to be made up of 

 an agglomeration of circular constituents, like wood 

 knots, and accordingly receive the name of " curly 

 cannel." The celebrated "Wigan cannel is somewhat 

 curly, but the most characteristic display of this curious 

 structure is aff'orded by the Flintshire curh' cannel, found 

 in the neighbourhood of Leeswood and Coed Talon, near 

 Mold. In many of the cottages of the thrifty colliers of 

 that neighbourhood a lump of this, carefully black- 

 leaded to display the curls, may be seen displayed in the 

 best Sunday room as a mantelpiece ornament. 



In this Leeswood seam the curly and the smooth 

 cannel lie in direct contact ; first about 2 feet of smooth 

 c:;nnel, below this about 18 inches of curly cannel, and at 

 the bottom of the curly 15 to 18 inches of bitujninous 

 shale. The roof shale overlying the smooth cannel is 

 black, but only slightly bituminous. The fact that has 

 puzzled me in studying this deposit is the absence of any 

 gr.^dation between the smooth and the curly ; the transi- 

 tion from one to the other is curiously sharp, although 

 they differ not only in structure and appearance, but also 

 in composition, the curly cannel being much richer in 

 gas-yielding constituents than the smooth in the pro]ior- 

 tion of about three to two (Dr. Andrew Fyfe's figures r.re 

 14,280 feet of gas per ton of curly, and 9,972 per ton of 

 smooth. "Wigan cannel 12,010). 



The cannel coals generally yield a larger quantity of 

 gas than ordinary bituminous coal. Both vary greatly 

 in this respect, as the above figures show, but takiug a 

 rough average, the quantity may be stated as about four 

 to three in comparing cannels with good gas coal. 

 ■ But the value of cannel in making gas for illuminating 

 purposes must not be measured merelj' by this. Com- 

 paring equal quantities, the value of the cannel gas 

 burnt in ordinary manner is far greater than that of gas 

 made from ordinary coal. There are good chemical 

 reasons for this, which I will endeavour to explain. 



The gas in both cases is a hydro-carbon ; but there are 

 hydro-carbons and hydro-carbons, i.e., compounds of 

 carbon and hydrogen, in which the elements exist in 

 varying jtroportions. For illuminating purposes we 

 may have too mtich carbon or too much hydrogen. In 

 the first case, the flame is lurid and smoky, unless 

 special devices are employed for supplying an excess of 

 oxygen or of heated air. In the second case, the flame is 

 deficient in luminositj-, is bluish and feeble, approaching 

 to that of an alcohol spirit lamp. 



There is a series of hydro-carbons which are technically 

 described as having a composition of CuH.,„x-:> smd another 

 series whose composition is represented by CuH„„_<j. 

 Those accustomed to chemical formulas will at once 

 perceive that the first of these all contain a larger propor- 

 tion of hydrogen than the second, that whatever figure 

 is represented by „, the number of equivalents of carbon 

 in the first must always fall below those of the hydrogen, 

 while in the second they must equal or exceed it. 



The first is the Marsh-gas, or paraffin series of hydro- 



