Mackintosh— The Sea y. Rivers. 155 



Dr. Voelcker states that these phosphatic nodules vary much in 

 composition, and that the average examples which he analysed are not 

 so valuable as those in the Upper Green-sand at Cambridge, which 

 generally contain 58 to 61 per cent, of phosphate of lime ; but it is 

 possible that some bands at Sandy may turn out to be much richer 

 than others, and, hence, by careful separation, the percentage may 

 be increased in the bulk to 54 or 56 per cent. The top beds are 

 richer than those worked at the depth of six feet. The phosphatic 

 matter contains a great deal of iron, derived, no doubt, from the 

 formation in which they are embedded, and which does not necessarily 

 belong to them, and many of them have a red tinge from this cause. 

 It may be observed, in conclusion, that this new locality at Sandy 

 has only been known for about two years, and has not long been 

 worked. 



III. — The Sea against Eiveks : or the Okigin of Valleys. 

 By D. Mackintosh, F.G.S. 



IN a former article^ I endeavoured to show that the more abrupt 

 inequalities of the earth's surface, so far as they consist of 

 escarpments with their associated phenomena, are chiefly due to the 

 former action of the sea. Before proceeding to consider the origin 

 of Yalleys, I find it necessary to refer to a statement I made in the 

 last article about Raised Beaches (page 69). My object was merely 

 to show that the preservation of numerous terraces in the Cretaceous 

 districts of Wilts and Dorset furnished an evidence of limited subaerial 

 denu.dation since these terraces were formed. The terraces to which 

 I alluded, principally occur at comparatively low levels ; and that 

 they are raised beaches can, I think, be clearly proved. But this is 

 not necessary, so far as the present controversy is concerned ; for the 

 preservation of terraces in gravel (more easily worn away than 

 chalk, according to the subaerialists), which all admit are either 

 raised sea-beaches, or glacial-lake-beaches (such as those of 

 Glenroy), afford an equally convincing proof of the impotence of 

 rain as a denuding agent.^ 



Additional Bemarlcs on tlie Denudation of the Weald. — Dr. Foster 

 and Mr. Topley, in their very learned paj)er already referred to,^ 

 admit that the gravel terraces of the basin of the Medway, some of 

 which are " well marTced," consist of " loose and incoherent deposits," 

 which the sea, had it been there, would siurely have swept away. 



1 Geol. Mag., vol. iii., No. 2, Feb., 1866. 



2 Mr. Codrington, F.G.S., informs me that lie has found made (/round on the 

 brows of several terraces of the class locally called Linchets, near "Warminster. That 

 the Belgse or other races may have increased the transverse horizontality of some of 

 these terraces, or may even have formed ■whole terraces, is easily conceivable ; but 

 that the thousands of terraces in the chalk districts of England were all fundament- 

 ally of human workmanship, is, I think, a theory involving such a series of improba- 

 bilities as to render it credible only to those who assign a most unwarrantable degree 

 of stability to the relative levels of land and sea. 



3 Quar. Jour. Geol. Soc, vol. xxi., No. 84, Nov., 1865. 



