266 



Prof. Rogers exhibited a large diagram showing the principal 

 features of Johnston's Physical Atlas. On it were displayed the 

 principal deposits of salt in the desert and semi-desert regions 

 of Northern Africa and Central Asia. He stated, that since the 

 last meeting he had discovered that Darwin, in his account of his 

 exploration of California and South America, had attributed the 

 occurrence of salt Lakes without outlet to the same cause that he 

 had suggested, the washings of rivers flowing into them. An- 

 gelot had proposed a similar hypothesis. Da Jean thought that 

 such Lakes, with a level below that of the sea, were more salt 

 than those above it, for the reason that they had probably been 

 cut off from it and had lost a part of their water by evaporation. 

 That this was not a true generalization was shown by the fact, 

 that the Great Salt Lake of California is almost a saturated solu- 

 tion of salt. The only point, therefore, in the hypothesis for 

 the origin of these Lakes, which Prof. Rogers said he could claim 

 as new, was the generalization that these salt Lakes only occur 

 where the evaporation exceeds the amount of rain ; and he con- 

 sidered this observation a contribution to geological science, as 

 aiding in illustrating the climate at different geological epochs. 



The saline deposits in the earth, even the rock salt, he believed 

 to have been formed by washings like the salt lakes of the pre- 

 sent time, and that they indicated the existence of dry climates 

 analogous to the present. 



Mr. Alger exhibited specin^ens of crystallized gold from 

 California. 



^ 



One of them was an octohedron of the dimensions of the 

 figure annexed, being ■f'-^ of an inch in diameter at the base of 

 the pyramid, and having four pretty regular faces, with three of 

 the solid angles perfectly formed to a point — two of the faces 

 were depressed, one of them into a very deep cavity surrounded 

 by a pretty uniform border parallel with the edges which unite 

 with the adjoining planes of the crystal. It appears as if the 

 crystal had been in a liquid state, and that soon after the outer 

 portion, or the surface of the planes, had congealed, the interior 



