452 PROF. J. W. JTTDD AND G. A. J. COLE ON THE 



As we have already seen, many writers have classified the easily 

 soluble forms as " tachylytes," and those only slightly acted on by 

 acids as " hyalomelanes." It says little, however, for the value of 

 this distinction, that different authors have placed the same rock in 

 different classes when judged by this standard. Thus Mohl's con- 

 clusions concerning the solubility of the different basalt-glasses 

 appear to be very different from those of Rosenbusch and other 

 writers. This solubility in acids appears to depend on so many 

 conditions, such as the more or less altered condition of the speci- 

 men, the fineness of the powder operated upon, the degree of con- 

 centration of the acid employed, the time and temperature of 

 digestion, &c, that it is not difficult to account for these striking 

 discrepancies. Penck, as we have already seen, has argued with 

 much force in favour of abandoning the distinction based on solu- 

 bility in acids * ; and in his later works Rosenbusch f himself 

 appears to have entirely abandoned it. 



In order to compare the behaviour of the several rocks here de- 

 scribed under the action of acids, we submitted weighed portions 

 of their powder to the action of concentrated hydrochloric acid for 

 a period of ten days, boiling them each day for some time. The 

 basalt-glass of the Beal left a residue of 81*6 per cent.; that of 

 Some of 70*79 ; that of Gribun 83*40 ; and that of Lamlash 

 88*37 1 ; while the rock of Screpidale, which was even attacked by 

 the cold acid, left a residue of only 57*82 per cent. In all cases 

 flocculent silica was separated, but in some in much greater quan- 

 tity than others. 



In order to compare these results with those given by other rocks 

 of the same class, we submitted a glassy lava of Hawaii and the 

 artificially fused rock of Rowley Regis to the same test. The 

 former left a residue of 50*57, and the latter of 58*7. 



Treated for the same time, but with only one boiling, Mr. Grant 

 Wilson § found that the Eskdale pitchstone left a residue of 83*2 

 per cent. We have determined that in the apparently similar 

 rock of Beinn Shiant the insoluble portion is, after daily boilings 

 during the same period, 81*17 per cent. 



Krukenberg || found the proportions dissolved from different 

 varieties of Hawaiian lava, which were digested in hydrochloric 

 acid from ten to sixteen hours, to vary from 49 to 62 per cent. ; 

 while B. Silliman, jun. ^, after similar experiments, obtained as 

 the insoluble proportion from 50 to 57 per cent. Cohen **, after a 



* Zeitschrift der deutschen geologischen Gesellschaft, vol. xxxi. 1879, p. 

 521, &c. 



t Mikroskopische Phvsiographie, vol. ii. p. 445 (1877), and Neues Jahrbuch 

 fur Min. &c, 1882, vol. "ii. p. 16. 



j Delesse found that, after boiling with hydrochloric acid, the glassy rock 

 of the Lamlash dyke gave a residue of 87"7 per cent., and the basaltic central 

 portion of 82 per cent. 



§ Proc. Roy. Phys. Soc. of Edinburgh, vol. v. p. 253. 



|| Mikrographie der Grlasbasalte von Hawaii. Tubingen : 1877, p. 3. 



*[ Memoirs of the Boston Soc. of Natural History, vol. i. p. 460. 



** Neues Jahrb. fur Min. &c. 1876, p. 746. 



