M. Agassiz on the Erratic Blocks (fthe Jura. 179 



It would be of great importance to ascertain if, towards the 

 poles, and in general where erratic blocks exist, the surface 

 which supports them is polished as in the Jura ; if every where 

 the large erratic blocks at a distance from the chains of moun- 

 tains whence they were derived are angular like those of the 

 Jura, and if, finally, they repose every where on a bed of round- 

 ed pebbles which are smaller the nearer they are to the polish- 

 ed surfaces. If the academy could obtain information on this 

 subjectfrom M. Dumont d'Urville, regarding the countries which 

 he is about to visit, a great step would be made towards the 

 determination of one of the most important questions in mo- 

 dern geology. 



2. On the Production of Crystals of Insoluble Substances by Arti- 

 ficial Means. By M. Gaudin. 



I have now the honour of submitting to the examination of 

 the academy (French Academy of Sciences) various specimens 

 of insoluble salts, in the form of very perfect microscopic 

 crystals, obtained by a process which I deem applicable to all 

 these substances, and capable of supplying them in crystals of 

 all dimensions. The process consists in placing certain solu- 

 tions in an artificial atmosphere ; as, for example, by placing 

 under the same receiver a cup containing moist carbonate of 

 ammonia, and a glass filled with a weak solution of any salt of 

 lime, barytes, lead, &c. At the end of some hours, there is 

 deposited upon the sides of the glass, crystals of a very pure car- 

 bonate, of these several bases, as may be seen in the specimens 

 which accompany this communication. 



With regard to crystals composed of elements which are 

 scarcely or not at all volatile, other arrangements must be had re- 

 course to. Thus, I have obtained the sulphate of barytes, for 

 example, by putting under the same receiver a flask of fuming 

 hydro-chloric acid, and a glass containing water, sulphate of 

 lime, and carbonate of barytes. 



Solutions of a salt of pure lime generally afford crystals under 

 the form of primitive rhombohedrons, or the principal modifica- 

 tions of that form; whilst the solutions of araffonite simultaneous- 

 \y give crystals in the form of Iceland spar, and in the form of 

 carbonate of barytes ; and, what is very singular, a solution of 



M 2 



