Geology and Mineralogy. 183 



condition that no amount of stirring or shaking or introduction of 

 foreign substances can make the solution crystallize and it 

 appears that this can only be done by the introduction of a crys- 

 tal, or the part of one, of the dissolved substance. This latter is 

 called the metastable state. 



Prof. Miers has made experiments to ascertain the exact limits 

 between these two states in a given solution, determining the 

 changing concentration by an optical method, and the tempera- 

 ture at which the change into the labile state occurs. Thus it was 

 found that in a solution of sodium nitrate containing 48 per cent 

 of tbe salt, if dust (containing assumably submicroscopic particles 

 of NaN0 3 or "germs") be not excluded, crystals make their 

 appearance on the surface of the liquid, grow, and sink, but 

 although they may be actively stirred about no new ones form 

 and the liquid remains in the metastable state till a temperature 

 somewhat below 16° is reached, when the labile region is entered 

 and a cloud of new crystals make their appearance. Thus in a 

 cooling supersaturated solution from which germs have not been 

 excluded there are two periods of growth ; one in which a com- 

 paratively small number of isolated crystals are growing regu- 

 larly and a subsequent period in which a shower of small crystals 

 is produced. If the rate of cooling is slow enough or the stirring 

 violent enough to keep the liquid in the metastable condition 

 there will be no second period or sudden production of small 

 crystals. These events were found to take place in all of the 

 solutions studied and the same process is suggested as an explana- 

 tion of the porphyritic texture found in igneous rocks ; "in a 

 silicate magma in all probability the temperature is sufficiently 

 high to be that of the metastable condition, the rate of cooling 

 sufficiently slow to keep the liquid in that condition for a consider- 

 able time and the viscosity sufficiently great to prevent the 

 growing crystals from sinking at once ; we have therefore all the 

 conditions favorable for the growth of porphyritic crystals ; 

 these must have generally originated throughout the liquid as 

 spontaneous nuclei if the magma entered the labile state, or may 

 have been started by inoculation or cooling at the margin if the 

 magma as a whole remained in the metastable state. In the 

 latter case suppose that further somewhat sudden cooling brings 

 the magma to the labile condition, then there will be a sudden 

 and spontaneous second growth of nuclei which will not be able 

 to attain the dimensions of the porphyritic crystals; we have here 

 all the conditions necessary for a second generation of one of the 

 constituents of the rocks." l. v. p. 



15. JBeitrdge zur chemischen Petrographie / von A. Osann, II 

 Teil, Analysen der Eruptivgesteine aus den J~ahren, 1881^-1900. 

 Pp. 266, 8vo. (Stuttgart, 1905.) — This work is in fact a continua- 

 tion of that of Roth, whose well-known Tabellen proved for 

 many years of such value to petrographers. It will be used 

 chiefly by Europeans, as in this country it is replaced by Wash- 

 ington's great work, but since in the latter the analyses are 



