President's Address. 17 



the zones of this series, especially the limestones, are trace- 

 able over the whole of the south of Scotland. This formation 

 is divisible into three sub-groups: (1.) a lower, comprising 

 limestones, sandstones, shales, coals, and ironstones; (2.) a 

 middle, containincj sandstones, shales, coals, and ironstones, 

 with no limestones ; and (3.) an upper, containing limestones, 

 shales, sandstones, coals, and ironstones. This doctrine is 

 perhaps better illustrated by this formation than by any 

 other Scottish deposit, because it affords remarkable examples 

 of the disappearance and reappearance of marine organisms 

 in particular areas according to the physical conditions which 

 prevailed. For example, the corals, crinoids, spirifers, pro- 

 clucti, etc., so abundant in the lower marine limestones, are 

 not met with in the middle subdivision, but the very same 

 species reappear in the Upper Limestone group. It is 

 apparent, therefore, that the marine organisms were com- 

 pelled to migrate to a more congenial habitat during the 

 deposition of the middle group, and as soon as the conditions 

 became favourable for their growth they reappeared, having 

 undergone but slight modification. The very same pheno- 

 mena are observable on a smaller scale in the respective 

 zones of the upper and lower subdivisions. When the suc- 

 cession is complete the following is the arrangement of the 

 strata in ascending order: — (1.) limestone charged with 

 ordinary marine fossils ; (2.) shales yielding stunted marine 

 forms ; (3.) sandstone ; (4.) fireclay with the roots of plants 

 which is overlain by a coal seam. In some cases one or 

 more of these members may be absent, but the others pre- 

 serve the same relative order. The limestones indicate 

 deposition in clear water where corals, crinoids, and other 

 marine organisms found a congenial habitat; the shales 

 point to the deposition of fine sediment, and the gradual 

 silting up of the sea bottom, conditions which were unfavour- 

 able to the growth of marine life ; while the sandstones in- 

 dicate a further shallowing of the sea and the distribution of 

 coarse sediment by marine currents. By degrees shallow 

 lagoons and mudflats were formed, suitable for the growth of 

 the vegetation now stored up in the seams of coal. The bed 

 of limestone which frequently forms the roof of a coal seam 



VOL. IX. B 



