Evans. — On Contact Metamorphism. 559 



derived from the dyke itself, as many instances are recorded 

 of such " white rocks." Microscopic observation should in 

 that case show traces of the original crystalline structure 

 of the igneous mass, though the component minerals would, 

 of course, be entirely altered. Chemically it is certainly 

 quite within the range of possibility that the reducing action 

 of the carbonaceous materials which must have abounded in 

 the clays near the dyke should be exercised through a distance 

 of several feet.) 



This " white rock " should prove of economic value. It 

 could well be used for road-metal, and when ground down (it 

 pulverizes fairly easily), and mixed with some lighter fire- 

 clay, slwuld make excellent firebricks, or a resisting lining for 

 furnaces. Similar material, ground to a fine sand, is much 

 used in England in the casting of brass. 



2. Plumbago (so-called) and Anthracite. 



Both of these seams fall really under the same heading. 

 Anthracite is generally defined as " the densest, hardest, and 

 most lustrous of all varieties of coal." It burns with very 

 little flame, but gives forch great heat — contains very little 

 volatile matter; splinters when heated, and ignites with 

 difficulty. Its colour is generally given as black. Its fracture 

 is lamellar, parallel to the bedding, and conchoidal in other 

 directions. Ure "^ mcludes three varieties — viz., anthracite 

 proper (defined much as above) ; culm, an impure shaly kind ; 

 and fossil coke, an American form, more compact than arti- 

 ficial coke, and supposed to be produced by the action of 

 trap rocks on anthracite {sic). 



The plumbago of the Brockley Mine is such an anthracite 

 shale, and will be reported on in detail (as also the outer white 

 rock) in a subsequent paper. 



The anthracite, or, as it might also apparently be named, 

 " fossil coke," is a really useful seam, and should prove of 

 great value as a fuel, either alone or mixed with the hy- 

 drous coals of the inner seams. It is rather coky in 

 appearance, is semi-lustrous, does not soil the fingers, is hard 

 to powder in a mortar, but, owing to the marked develop- 

 ment of the lamellar structure, and the existence in places 

 of a columnar structure at right-angles to the bedding-planes, 

 easy to break into lumps. It gives very little flame, contains 

 but little over 1 per cent, of sulphur, and, except for the some- 

 what high percentage of water, has every characteristic of a 

 true " fossil coke," or stone coal.t 



* " Dictionary of Arts, &c.," vol. i., p. 744. 



f For analytical details of these coals, see below Art. LIV., "An- 

 alyses of New Zealand Coals." 



