170 



In framing any conjecture as to the length of time correspond- 

 ing to the formation of a group of strata at any particular locality, 

 as the Joggins, we would of course ascribe but a small value in 

 years to such masses of deposit as thus prove themselves to have 

 been hastily accumulated at the spot where they are found. But 

 on the other hand we should be careful not to apply the same 

 measure of rapid accretion to those associated beds of shale, lime- 

 stone, coal, and even sandstone, which give intrinsic evidence of 

 having been tranquilly and slowly deposited. We should also 

 keep in view the important fact, that while one part of the column 

 of strata whose chronology we are studying has been thus rapidly 

 built up, by the materials swept into it from a neighboring quar- 

 ter, other parts of the same column have been reduced in thick- 

 ness, or even wholly removed, by similar local actions in the 

 opposite direction ; and that therefore the strata as they stand give 

 us the measure of a time much less than that in which, as a group, 

 they were actually deposited. 



Prof. Rogers next proceeded to compare the Lower Carbonife- 

 rous rocks of Nova Scotia and New Brunswick, which he had 

 lately in part examined, with certain groups of strata holding a 

 corresponding geological position in the Appalachian belt of the 

 United States. 



Early in the geological surveys of Virginia and Pennsylvania, 

 it was found that two groups of strata of great thickness were 

 interposed between the series of arenaceous red rocks forming the 

 top of the Devonian and those massive conglomerates and sand- 

 stones which usually mark the base of the true coal measures. 



Of these intervening masses, the Lower Group, consisting of 

 conglomerates, sandstones, and sandy slates, and shales, usually of 

 a brownish and greenish gray color, abounds in impressions of 

 Lepidodendra and other terrestrial plants allied to those of the 

 true coal measures, although not in general identical with them, 

 and includes in some of its outcrops one or more considerable 

 seams of coal. 



The Upper Group, exposed along the northeastern margin of 

 the coal region of Pennsylvania, consists of a great thickness of 

 red shales and sandstones, passing upward into buff argillaceous 

 sandstones, and including at some points a few calcareous layers, 



