170 



This is the last of a series of communications by the author on 

 the same subject ; and in this he endeavours to classify all the organic 

 remains found by him in the Plymouth limestone. In this arrange- 

 ment there are enumerated several genera of Polyparia, including 

 Spongia?, Stylina, Caryophyllia, Turbinolia, &c. ; several species 

 of* Crinoidea, and genera of Conchifera and Mollusca. 



After a detailed description of many species in each of the above 

 classes, the author concludes, that as the number of Zoophytes 

 bears a very large proportion to that of the Bivalves and Uni- 

 valves, the Plymouth limestone must be considered to be one of 

 the earliest deposits. But he states that great obscurity still in- 

 volves the relative distribution of these animals in their order of 

 superposition. 



A paper was afterwards read, " On the gradual Excavation of 

 the Valleys in which the Meuse, the Moselle, and some other Rivers 

 flow;" by G. Poulett Scrope, Esq., F.G.S. F.R.S. 



The paper commences by a remark on the value which would 

 attach to a test by which any one valley could be ascertained to be 

 the result either of a rapid and violent, or of a slow and gradual ex- 

 cavatory process ; since the forces of aqueous erosion are of a 

 general nature, and while in activity in one river channel, were 

 probably not idle in others. Such a test has been pointed out by 

 the author in central France, where lava-currents which have flowed 

 into valleys at intervals of time, appear now at different heights 

 above the actual river-bed, marking the successive steps of the pro- 

 gress of excavation. 



The author finds another equally valuable test in the extreme 

 sinuosities of some valleys. Any sudden, violent, and transient rush 

 of water of a diluvial character, could only produce straight trough- 

 shaped channels in the direction of the current, but could never 

 wear out a series of tortuous flexures, through which some rivers 

 now twist about, and often flow for a time in an exactly opposite 

 direction to the general straight line of descent, which a deluge or 

 debacle would naturally have taken. Curvatures of this extreme 

 kind are frequent in the channels of rivers flowing lazily through 

 flat alluvial plains ; and the author shows the mode in which the 

 curves are gradually deepened and extended, till the extreme of 

 aberration is corrected at once, and the direct line of descent re- 

 stored, by the river cutting through the isthmus, which separates 

 two neighbouring curves. 



But examples must be infinitely rarer of whole valleys charac- 

 terized by extreme sinuosity ; because, in the author's opinion, the 

 frequent shiftings of the channels of streams tend to obliterate their 

 windings, and reduce the sum of the several successive excavations 

 or valley to a more or less straight form. Still there are occasional 

 instances where the bias of the river, or direction of its lateral force 

 of excavation, has remained so constant as to give to the valley it- 

 self the utmost degree of sinuosity. 



The author quotes the valley of the Moselle between Berncastle 

 and Roarn, excavated to a depth of from 600 to 800 feet through 



