A. C. Seward — Variation in Sigillarics, 213 



rocks, from the depressions now occupied by the Gulfs of St. Law- 

 rence and Maine, which have, however, been more or less affected 

 by terrestrial movements. 



The length of time required to excavate the channels of these 

 great rivers commenced as far back as the Palaeozoic days. How- 

 ever, the culmination of that of the Mississippi was not until in the 

 later Tertiary, before the Pleistocene period. As the St. Lawrence, 

 now submerged to a depth of over 1200 feet for a distance of 800 

 miles, is mostly cut out of rocks of the Palaeozoic group, except 

 a belt of the Triassic (across the lower portion, more or less involved 

 in mountain uplifts), its antiquity must be very great. The culmi- 

 nation was also probably in the later Tertiary era, like that of the 

 Mississippi, and tbe channels on the California coast, for there are 

 submerged Tertiary rocks off the coast of Massachusetts and New- 

 foundland, at elevations much higher than the beds of the old 

 channels. 



Although the excavating forces took so many periods to form 

 the valleys, and required a high continental elevation, yet the 

 extreme altitude of over 1800 feet appears to have been of com- 

 paratively short duration, for otherwise the deep chasms in which 

 the submerged channels terminate would have extended farther 

 inland than we find them, and would have been headed by more 

 gentle slopes, in place of precipitous cliffs, over which the waters of 

 the former rivers were precipitated in great cascades. In the fjords 

 of Norway, merging into rapidly contracting valleys, or headed by 

 great vertical walls, hundreds of feet in height, having the structure 

 named cirques, may be seen to-day the counterpart of the coast of 

 the American continent, when its marginal plateaux stood over 

 3000 feet higher than at present ; yet Norway stood once much 

 higher than now, but was afterwards submerged, from which 

 depression it has only recently been re-elevated so that its plateaux, 

 close upon the sea, rise to three or four thousand feet, and its 

 mountains still higher. 



The old hydrography is more or less disturbed by warpings of 

 the earth's crust, which, however, do not obscure the valleys, 

 although rendering the features somewhat more complex. The 

 amount of distortion has yet to be determined. 



IV. WOODWARDIAN LABORATORY NOTES. 



By A. C. Seward, M.A., F.G.S., 

 St. John's College, Cambridge. 



1. — Specific Variation in Sigillaria:. 



~\j~0 better example could perhaps be quoted of fossil plants which 

 _LAI have given rise to a number of species and even genera 

 founded on imperfect and fragmentary specimens than the Sigillarice 

 and Lepidodendra. 



Our knowledge of the internal structure of these plants has so 

 far increased in recent years that we are now able to appreciate the 

 differences between stems or branches of plants which have been 



