Sir James Hall on the Contolidation of' the Strata. 3 



substance. Hence the crystals may mutually interfere with 

 each others forms, and we may as naturally expect to see quartz 

 moulded on crystals of feldspar, as the reverse.'" 



Sir James then discusses the question, as to the effects of 

 slow cooling, in preventing the return of the fused substance 

 to a state of glass — and then advances the following very in- 

 genious views, which are incapable of abridgement. 



" If quartz, mica, feldspar, schorl, garnet, &c. happen to 

 be melted together, the most fusible substance of them all may 

 be considered as the menstruum in which all the rest are dis- 

 solved, and we may suppose, that these various dissolved sub- 

 stances may differ amongst themselves in their properties of 

 solution, as salts differ from one another, so that some of them 

 may be more soluble, when very much heated, than when it 

 is comparatively cold, and others may be as soluble in it, when 

 little warmer than its point of congelation, as when raised to a 

 much higher temperature. If then we say, for example, that 

 the congealing point of the solvent is 1000 degrees of Fahrenheit, 

 and if the solution is at the temperature of 2000, we may con- 

 ceive one portion of the matter dissolved as held by the sim- 

 ple dissolving power of the menstruum, and another portion as 

 held by means of its elevated temperature. When, therefore, 

 a mass of this kind is allowed to cool very slozcly, as we may 

 suppose must be the case with liquid granite in the bowels of 

 the earth, those substances held in solution by the heat of the 

 solvent, will first separate, and, being formed in a liquid, will 

 assume their crystalline forms with perfect regularity ; where- 

 as those substances, which were held by the menstruum, simply 

 as a fluid, will not separate till the congelation of the solvent 

 itself takes place, when the crystals of the various substances 

 will intermix and confound the regularity of form, which each 

 would have assumed, if left to itself. In this manner, one of 

 the most common kinds of granite will be produced, consisting 

 of perfect crystals of schorl, mica, or garnet, inclosed in a 

 confused mass of quartz and schorl." 



These theoretical views, whether just or otherwise, it must be 

 admitted, are highly curious and philosophical, and are well de- 

 serving of being put to those experimental tests, which no man 

 knows better how to devise than Sir James himself: And as 



