10 THE GEOLOGICAL HISTORY OF PLANTS. 



those of the containing rock. Sir William Logan states * 

 that "the deposits of plumbago generally occur in the 

 limestones or in their immediate vicinity, and granular 

 varieties of the rock often contain large crystalline plates 

 of plumbago. At other times this mineral is so finely 

 disseminated as to give a bluish-grey colour to the lime- 

 stone, and the distribution of bands thus coloured seems 

 to mark the stratification of the rock." He further 

 states: "The plumbago is not confined to the lime- 

 stones ; large crystalline scales of it are occasionally dis- 

 seminated in pyroxene rock, and sometimes in quartzite 

 and in feldspathic rocks, or even in magnetic oxide of 

 iron." In addition to these bedded forms, there are also 

 true veins in which graphite occurs associated with cal- 

 cite, quartz, orthoclase, or pyroxene, and either in dis- 

 seminated scales, in detached masses, or in bands or layers 

 "separated from each other and from the wall-rock by 

 feldspar, pyroxene, and quartz." Dr. Hunt also men- 

 tions the occurrence of finely granular varieties, and of 

 that peculiarly waved and corrugated variety simulating 

 fossil wood, though really a mere form of laminated 

 structure, which also occurs at Warrensburg, New York, 

 and at the Marinski mine in Siberia. Many of the veins 

 are not true fissures, but rather constitute a network of 

 shrinkage cracks or segregation veins traversing in count- 

 less numbers the containing rock, and most irregular in 

 their dimensions, so that they often resemble strings of 

 nodular masses. It is most probable that the graphite of 

 the veins was originally introduced as a liquid or plastic 

 hydrocarbon ; but in whatever way introduced, the char- 

 acter of the veins indicates that in the case of the greater 

 number of them the carbonaceous material must have 

 been derived from the bedded rocks traversed by these 

 veins, to which it bears the same relation with the veins 



* "Geology of Canada," 1863. 



