154 THE CARBONIFEROUS SYSTEM. 



are immense quantities of fossil plants, most of them trunks of trees 

 confusedly intermingled and flattened more or less by pressure ; others 

 long cylindrical reed-like stems (Calamites), or immense creeping roots 

 dotted all over with pits from which their rootlets sprang (Sligmariaz). 

 In most of these fossils the bark is converted into hard shining coal, 

 but the wood has decayed away, and the hollow cavity left within the 

 bark, has been filled with sand now hardened into stone like that 

 without. This is a distinct process from petrifaction properly so 

 called, in which the minute cells of the wood become so filled with 

 mineral matter that the minutest parts of the structure are preserved. 

 Some of the gray sandstones of this part of the section are of great 

 thickness, and in them are the most important quarries of the Joggins 

 grindstones, which are exported to all parts' of the United States. 

 These grindstones have been formed from beds of sand deposited in 

 such a manner that the grains are of nearly uniform fineness, and they 

 have been cemented together with just sufficient firmness to give 

 cohesion to the stone, and yet to permit its particles to be gradually 

 rubbed off by the contact of steel. A piece of grindstone may 

 appear to be a very simple matter, but it is very rarely that rocks are 

 so constituted as perfectly to fulfil these conditions, and hence the 

 great demand for the Joggins stone. 



This part of the section suggests many interesting inquiries respect- 

 ing the mode of formation of some of its beds, but I postpone these 

 till Ave arrive at those portions which show coal measures, properly so 

 called, on a somewhat larger scale. 



Proceeding along the coast, we find that the strata last described 

 are overlaid by a series amounting to 2082 feet in vertical thickness, 

 and differing from the last group of beds in containing fewer gray 

 sandstones, no coal-seams or bituminous limestones, and comparatively 

 few fossil plants, and these but imperfectly preserved. This series, 

 then, consists in great part of reddish shales and reddish and gray 

 sandstones. These, and indeed the greater part of the rocks com- 

 posing the part of the section we have examined, must originally have 

 consisted of beds of reddish sand and mud, spread over the bed of that 

 ancient Carboniferous sea once tenanted by the shells of the Napan 

 limestone, much in the same manner that layers of mud are now 

 deposited in the Bay of Fundy. 



We have now, after passing over beds amounting altogether to the 

 enormous thickness of 7636 feet, reached the commencement of the 

 true coal measures, or that part of the section which was examined in 

 detail by Sir Charles Lyell and the writer in 1852 and 1853. Owing 

 to the comparative softness of the rocks of the last group described, 



