President of the Geological Society for 1850. 25 



The minute study of the structure and organic contents of 

 strata of various ages, has made us of late years more and 

 more familiar with the hypothesis of a slow sinking of the 

 ancient floor of the ocean going on while it was receiving 

 repeated accessions of sediment. We must not forget that 

 in all such cases a solid foundation of subjacent rock of un- 

 known depth, and perhaps much older than the newly super- 

 imposed deposit, is undergoing simultaneously a change of 

 position, and that rocks still lower are undergoing, whether 

 by cooling or crystallizing, a change of structure. These 

 very gradual movements are quite as remarkable in the pa- 

 laeozoic as in the tertiary periods. By consulting the " Me- 

 moirs of the Geological Survey of Great Britain," you will 

 learn that in Wales, and the contiguous parts of England, a 

 maximum thickness of 32,000 feet (more than six miles), of 

 carboniferous, Devonian and Silurian beds, has been mea- 

 sured, the whole formed whilst the bed of the sea was con- 

 tinuously and tranquilly subsiding. In illustration of a 

 movement of the same kind, I need scarcely remind you of 

 the coal-measures of South Wales, with their numerous 

 under-clays, each containing Stigmaria, a phenomenon to 

 which Mr Logan first drew our attention. Mr Binney of 

 Manchester has since proved to us that all these Stigmariae, 

 found in the floor of every coal-seam, are the roots in situ 

 of fossil trees, chiefly of the genus Sigillaria, and that they 

 are occasionally attached to their stems or trunks, — a con- 

 clusion fully confirmed by the more recent observations of 

 Mr Richard Brown on the coal-fields of Nova Scotia. Sir 

 Henry De la Beche also, in his paper on the rocks of South 

 Wales and the south-west of England, confirms these state- 

 ments, and shows that subsidences of vast amount took place 

 slowly during the accumulation of the palaeozoic strata, the 

 sea all the while remaining shallow, in spite of a depression 

 of one or two miles. Still later. Professor John Phillips, in 

 the second volume of the same " Survey," has pointed out 

 analogous phenomena in the old red sandstone of the Forest 

 of Dean ; and these strata, 7000 feet thick, are described as 

 liaving been formed in a sea of moderate depth. Fossil 

 corals and shells imbedded as they grew, or ripple-marked 



