No. 31,-1885.] 



PLUMBAGO. 



no 



south to north of the continent. Near Darjiling, in the 

 district of British Sikhim, there is a very curious result of the 

 crushing of coal by slipping mountain strata. The friction of 

 the process must have evolved heat to a considerable degree, 

 but instead of anthracite the crushed coal was converted into a 

 substance which the geologists have named " semigraphite," 

 -—a hint here, probably, of the agencies at work in producing 

 true graphite in India, Ceylon, and elsewhere. Indeed real 

 graphite has been found as the result of the alteration of a 

 coal seam by intrusive basalt, as atNew Cumnock in Ayrshire. 

 So writes Geikie, but like other geologists he notices graphite 

 as being amongst the older formations, where it occurs in dis- 

 tinct lenticular beds and also diffused in minute scales, 

 Geikie's full description of the mineral is given in a note.* In 



* Graphite. — Rarely crystallized in hexagonal forms, usually granular 

 scaly, or compact. H. 0'5 -— l'O. Gr. 1*9— 2'3. Nearly pure carbon, 

 but generally with at least 1 or 2 per cent, of silica, lime, iron, or other 

 impurity. Under the microscope, opaque ; appearing velvet black 

 with reflected light. Found chiefly in ancient crystalline rocks, as 

 gneiss, mica-schist, granite, &c. ; some of the Laurentian limestones of 

 Canada being so full of the diffused mineral as to be profitably worked 

 for it; in rare instances coal has been observed changed into it by 

 intrusive basalt (Ayrshire). Probably in most cases the result of the 

 alteration of imbedded organic matter, especially remains of plants ; 

 occasionally observed as a pseudomorph after calcite and pyrites, and 

 sometimes enclosing sphene and other minerals. 



Graphite is little affected by percolating water, hence it is not a 

 replacement mineral. But Vom Rath has described an example from 

 Westphalia, where calcite has been partially replaced externally by an 

 encrusting pseudomorph or graphite. 



Geikie also states : " The opinion of the organic nature of Eozoov, 

 has been supposed to receive support from the large quantity of 

 graphite found throughout the Archaean rocks of Canada and the 

 northern parts of the United States. This mineral occurs partly in 

 veins, but chiefly disseminated in scales and lamina? in the limestones 

 and as independent layers. Dr. Dawson estimates the aggregate amount 

 of it in one band of limestones in the Ottawa district as not less than 

 from 20 to 30 feet, and he thinks it is hardly an exaggeration to say that 

 there is as much carbon in the Laurentian as in equivalent areas of the 

 carboniferous system. He compares some of the pure bands of 

 graphite to beds of coal, and maintains that no other source for their 

 origin can be imagiDed than the decomposition of carbon dioxide by 

 living plants. In the largest of three beds of graphite at St, John he 

 has found what he considers may be fibrous structure indicative of the 

 existence of land-plants," 



