472 



Dr. M. C. Stopes. On the Four Visible 



•These four distinguishable ingredients, all of which, in varying quantities,, 

 are to be found in most ordinary bituminous coals, I name provisionally as 

 follows : — 



(i) Fusain* The equivalent of " mother of coal," " mineral charcoal," 



etc., of various authors. 



(ii) Durainf The equivalent of " dull " hard coal of various authors, the 



" Mattkohle " of Germans, etc. 



(iii) Clarainf"^ Together the equivalent of "bright" or glance coal of 



I various authors, the " Glanzkohle " of Germans. Some- 

 Y times the " bright " coal of an author seems to bfe the 

 I vitrain only. 



(iv) Vitrainf J (Conchoidal fracture, brilliant in appearance.) 



[These names, I am fully aware, do not represent chemical entities (with 

 the possible exception of vitrain), but they do represent tangible entities of 

 the same useful order as " jet," " granite," or " cheese."] 



The generally " streaky " or banded nature of a seam of coal is of varying 

 orders of magnitude, and as. one magnifies a banded piece of coal more and 

 more it becomes increasingly apparent how finely laminated it may be. 

 Hence, a diagram of the arrangement of the bands natural size, magnified by 

 4, and another by 10 or even 20 diameters, may all show essentially similar 

 lenticular intercalated lamellar structure. 



In hand specimens sometimes bands two to four inches or more in thick- 

 ness may be all " dull " coal, little, if at all, streaked with " bright ; " and 

 perhaps above or below that may be three or four inches of glossy " bright," 

 •only finely streaked with dull. Most coals, however, are more mixed than 

 that, and the average " dull " band is from a quarter of an inch or more in 

 thickness, and is all through visibly streaked with fine lenticels of " bright," 

 while the " bright " portions are streaked with very variable bands of " dull." 

 Both the " dull " and the " bright," both the fusain and the vitrain, are all 

 essentially lenticular masses ; these are often so horizontally extended and so 

 thin that they create the impression of being fine horizontal bands. With 

 very few exceptions they lie approximately parallel to the bedding, plane of 

 the deposit. The fusain is the least regular in its arrangement, but on the 



* The French name, adopted into English by J. J. Stevenson (1911-13) and Stopes and 

 Wheeler (1918), to replace our native unwieldy and misleading names "mother of coal" 

 and " mineral charcoal." 



+ The first use of new terms suggested by the present author, and each based on a 

 Latin root descriptive of the substance and terminated in -ain to match fusain. The 

 latter word is a French word used by geologists in a specialised sense. It is based on 

 the Latin fusus, and its application to the " mineral charcoal " came about in a circuitous 

 way. The roots I have chosen are obviously and directly descriptive. 



