1853.] DAWSON COAL-MICASURES, NOVA SCOTIA. 11 



the results of these changes in the Joggins section may be accounted 

 for by the fact, sufficiently proved by other evidence, that the same 

 beds of clay have been subjected to the action of vegetable matter, 

 decomposing first in fresh water and then in the sea, or at least in 

 brackish water*. In connection with these changes of colour, it 

 occurs here as elsewhere that fossils are rare in the reddish beds ; 

 the absence of vegetable matter being in fact the cause of the retention 

 of the red colour. 



Black bituminous or carbonaceous shales are more rare at the 

 Joggins than at Sydney, and much more so than at Pictou ; and 

 this, as well as the great number and small aggregate thickness of 

 the coal-seams, probably indicates greater rapidity of change, and 

 more frequent invasions of sedimentary matter, than at those localities. 

 Several beds of shale, by the presence of great quantities of shells of 

 Modiola and Ci/pru^ with much organic matter, pass into a tough 

 calcareo-bituminous rock, and finally into hard bituminous lime- 

 stone. It is observable that the passage from common argillaceous 

 shales to these Modiola-shales is often very gradual, the ModiolcB 

 becoming more and more rare in ascending through several feet of 

 shale. In these cases these shell-fish probably died out as their 

 abode became invaded by muddy sediment. 



Beds of clay containing roots of plants in situ, and destitute or 

 nearly destitute of lamination, are designated in the Section " Un- 

 derclays." As these are fossil soils, they will be more properly 

 considered in connection with the vegetable matter which accumulated 

 upon them. 



2. Sandstone. — The line of distinction between these and shales is 

 of course somewhat arbitrary, and I observe that many fine-grained 

 beds, named sandstones in our notes, appear in Mr. Logan's section 

 as argillaceous shales. None of the sandstones observed by us were 

 very coarse, and most of them are argillaceovis, though a few, espe- 

 cially the thicker grindstone reefs, are very purely siliceous and made 

 up of sharp and uniform fine grains of sand. A few beds are hard, 

 with a calcareous cement. The colour varies from the bluish-grey of 

 the celebrated Joggins blue grindstones, to fawn and buff, the latter 

 being usually a superficial discoloration occasioned by iron pyrites. 

 We observed no beds of distinctly red colours, although these abound 

 both above and below the part of the section examined by us. Mr. 

 Logan notes a few beds as of reddish colours. These we have probably 

 not separated from the chocolate shales, which are in some places 

 highly arenaceous. 



Regularly laminated and ripple-marked sandstones are com- 

 paratively rare, especially in the central part of the section ; and 

 most of the beds present various irregularities, such as rapid changes 

 of thickness, false stratification, and large undulations of the sur- 

 faces. This is probably a consequence of the littoral origin of these 

 beds, which is further indicated by the occui-rence of unworn trunks 

 of trees, fragments of vegetable matter scattered over the strata-sur- 



* Dawson on the colouring matter of Red and Grey Sandstones, Quart. Jouru. 

 Geol. Soc. vol, V. p. 25. 



