Ml* J. D. Dana on the Volcanoes of the Moon. 27 



In these remarks we have spoken of the lavas as consisting 

 mainly of feldspar and augite, their more common constitu- 

 tion ; but v^^e use the terms in a general sense, understand- 

 ing by feldspar one or another of the feldspar family of mi- 

 nerals, and by augite the remaining fusible material, whether 

 ordinary augite (silicate of lime, magnesia and iron), or sili- 

 cates of one or more of these bases or alumina in other com- 

 binations. 



There is some difficulty in applying this hypothesis to par- 

 ticular cases, on account of our ignorance of the actual fusi- 

 bilities of the materials of the lava, in the condition in which 

 they are placed ; for we know that an infusible mineral may 

 be held in fusion under certain circumstanceSj or with cer- 

 tain mineral associations, far below the temperature at which 

 it fuses ; or, previous to the commencement of cooling, it may 

 be in some other combination. 



We should infer that the process which separates the feld- 

 spar, would also separate any excess of the more infusible 

 mineral quartz. This may not follow : still it is a remark- 

 able fact, that the quantity of quartz contained in trachyte 

 is often in great excess, as analyses have shewn. But why 

 is Jiot the infusible mineral chrysolite also detained \ The 

 fact appears to be, that it is of subsequent formation. The 

 small proportion of silica it contains implies a deficiency of 

 this substance ; while, as we have stated, in the feldspathic 

 rocks there is often an excess. It may, therefore, under cer- 

 tain circumstances proceed from the basaltic material, for its 

 elements are the same in different proportions. Subsequent 

 investigations may give us more light on this point. 



The general principle which we have above brought for- 

 ward, is well illustrated in the fact, that the scoria or sur- 

 face glass of any vent, where it occurs, is the most fusible 

 part of the lava, consisting, in general, of ferruginous or al- 

 kaline silicates, and containing no magnesia. On account of 

 the diminished heat, this material alone remains sufficiently 

 fluid to be inflated and borne up to the surface by the rising 

 vapours : and this takes place in spite of superior gravity. 



this last supposition borne out by any existing example of subaerial volcanic 

 action ? 



