28 THE DAWN OF LIFE. 



containing rock. Sir William Logan states* that 'the de- 

 posits 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- 

 gray colour to the limestone, 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 

 limestones ; large crystalline scales of it are occasionally dis- 

 seminated in pyroxene rock or pyrallolite, 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 calcite, 

 quartz, orthoclase, or pyroxene, and either in disseminated 

 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 mentions the occurrence of finely 

 granular varieties, and of that peculiarly waved and corru- 

 gated variety simulating fossil wood, though really a mere form 

 of laminated structure, which also occurs at Warrensburgh, 

 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 countless 

 numbers the containing rock, and most irregular in their 

 dimensions, so that they often resemble strings of nodular 

 masses. It has been supposed that the graphite of the veins 

 was originally introduced as a liquid hydrocarbon. Dr. Hunt, 

 however, regards ifc as possible that it may have been in a 

 state of aqueous solution ; f but in whatever way introduced, 

 the character 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, 

 while there can be no doubt that the graphite found in the 

 beds has been deposited along with the calcareous matter or 

 muddy and sandy sediment of which these beds were originally 

 composed. 



* Geology of Canada, 1863. 

 t Report of the Geological Survey of Canada, 1866. 



