76 Geological Society : — 



wliich, when the incident rays are parallel, must be taken for the 

 secondary caustic in the place of the Cartesian, which, for the parti- 

 cular case in question, passes off to infinity. In the qourse of the 

 memoir, the author reproduces a theorem first given, he believes, 

 by himself in the Philosophical Magazine, viz. that there are sL\ 

 different systems of a radiant point and refracting circle which give 

 rise to identically the same caustic. The memoir is divided into 

 sections, each of which is to a considerable extent intelligible by 

 itself, and the subject of each section is for the most part explained 

 by the introductory paragraph or paragraphs. 



GEOLOGICAL SOCIETY. 

 [Continued fi-om vol. xii. p. 482.] 

 November 19, 1856. — Col. Portlock, President, in the Chair. 

 The following communications w^ere read : — 



1. "On the Occurrence of crystallization in Stucco." By Dr. 

 Buist. Communicated by Sir R. I. Murchison, F.G.S. 



The author remarked that in the spring of 1848 a quantity of 

 fragments of plaster-castings which he had thrown out of doors 

 ■were, after exposure to the atmosphere and rain, before long covered 

 with crystals of selenite. Specimens, communicated by Dr. Buist, 

 were exhibited, together with calcareous casts of shells and tubular 

 bodies, from the upraised beaches of the coast of India, which the 

 author sent and described as exhibiting other interesting instances 

 of alteration in molecular arrangement. 



2. " On the Occurrence of AUophane at Charlton, Kent." By 

 J. Morris, Esq., F.G.S. 



The author described the circumstances under which allophane 

 occurs in the fissures of the Chalk near Woolwich, and pointed out 

 that its formation was evidently of later date than the disturbances 

 of the Chalk and the overlying Thanet sands, and of the determina- 

 tion of the present physical features of the district. 



An analysis made by Mr. Dick gave the following composition : — 



Silica 18'89, alumina 33'52, water 42"73, lime 1'67, carbonic acid 

 2'51, organic matter, a trace. 



Mr. Morris suggested that this allophane was probably due to 

 the percolation of atmospheric waters through the superficial de- 

 posits, and the sands immediately overlying the chalk ; and he 

 pointed out that probably Scarbroite (or Kollyrite), Pholerite, and 

 several other allied hydrosilicates of alumina should not be regarded 

 as distinct mineral species ; the slight variation in their chemical 

 formulae having been probably determined by the local conditions 

 under which each of them originated. 



3. " On the Red Sandstones and Quartzites of the North-west of 

 Scotland." By Prof. James Nicol, F.G.S. 



The district described in this communication stretches, N.N.E. 

 and S.S.W., from Cape Wrath and Durness to Sleat, the southern 

 portion of the Isle of Skye, with an extent of more than 100 miles 

 and a breadth of from 15 to 30 miles. Its structure was illustrated 

 by transverse sections afforded by Loch Broom, across the centre of 



