238 Dr Charles Daubeny on the 



klase, minerals of analogous constitution, though of different 

 external characters, and with different relative proportions of 

 the two alkalies present in them. 



In trachytes, properly so called, there would appear to 

 have been such an accession of alkali and of earths, that the 

 whole of the silica entered into combination, and consequently 

 no quartz exists in the rock. 



But when we proceed to the lava currents which have been 

 emitted from actual volcanoes, or to the analogous trap for- 

 mations which are regarded as the effects of submarine erup- 

 tions, we find a still further diminution in the proportion of si- 

 lica, indicated by the substitution of labradorite for orthoklase, 

 or, in other words, the presence of one atom of felspar in- 

 stead of three, coupled with that of hornblende or augite,* 

 in both which minerals the silica bears a still smaller pro- 

 portion to the base with which it is combined. 



In these last minerals two new elements also make their 

 appearance, which are seldom or never present, except in 

 small quantities, in granite or in trachyte — I mean lime and 

 magnesia ; thus evincing already a change, either in the na- 

 ture of the igneous operations, or in the materials upon which 

 they were exerted. 



Thus the modern lavas of mount Etna have been deter- 

 mined by Lowet to consist of an intimate mixture of labra- 

 dorite and of augite ; and a lava which had recently flowed 

 from Stromboli was ascertained by Abicli to possess the same 

 composition. 



Greenstone, or dolerite, is composed of nearly the same 

 materials, its compactness being merely the effect of the 



* Hornblende is R Si + R^ Si^, where R is generally lime, but sometimes 

 protoxide of iron, or soda ; and B? is generally magnesia, but sometimes pro- 

 toxide of iron. In some hornblendes the silica seems to be partially replaced 

 by alumina. Bomdorff, Augite is ll^ Si^, where R is either lime, magnesia, 

 protoxide of iron, or protoxide of magnesia. The silica is sometimes replaced 

 by alumina, as is the case also in hornblende. See Rammelsberg's Dictionary 

 of Mineralogy, Berlin, 1841. 



t Jameson's Journal, 1837. 



