170 [June 19, 



the optical constants of the alloys of copper and zinc, which cannot 

 be given in an abstract. 



6. In the details of the several experiments, the author calls atten- 

 tion to several remarkable laws, or indications of laws, which appear 

 to him to require some notice from theorists. 



a. When the azimuth of the incident beam is less than the circular 

 limit, the axis major of the reflected ellipse, at the principal incidence, 

 lies in the plane of incidence ; but when the azimuth is greater than 

 the circular limit, it is perpendicular to the plane of incidence, and 

 as the incidence varies, the axis major twice approaches to a minimum 

 distance from that plane. 



b. There appears to the author to be some indication in the expe- 

 riments on metals, that the quantity known to theorists as (jj is 



not a function of the incidence only ; a conclusion which, if correct, 

 would require the intervention of a third wave suppressed, or some 

 such theoretical supposition, to account for it. 



XII. " On the Loess of the Valleys of the South of England and 

 of the Somme and the Seine." By JOSEPH PRESTWICH, 

 Esq., F.R.S. Received June 19, 1862. 



(Abstract.) 



In this paper the author takes up and discusses a point connected 

 with the former inquiry, but postponed in the paper he read before 

 the Royal Society in March last, a recent visit to France having led 

 him to form a conclusion with regard to the origin of the Loess 

 sooner than he then expected. 



On that occasion he referred the loam and brick- earth, with land 

 and freshwater shells, which occurs in the valleys and on many of the 

 hills in the South of England and North of France, to temporary in- 

 undations of the old rivers. On the present occasion he shows that 

 this deposit is intimately connected with the origin of the river- 

 valleys and with the fluviatile high- and low-level gravels described in 

 his last paper. 



