CROSS, JDDINGS, PIRSSON, WASHINGTON 



sum total we believe that it is less, or at all events no greater, 

 than in any system heretofore proposed. But in regard to this 

 each must judge for himself. 



Objection may also be raised to this method of classification, 

 that it entails a greater amount of labor upon the petrographer 

 than those now in use. Perhaps there are some who, recalling 

 the possibility of glancing through a microscope and noticing 

 the presence of striated feldpars, with one or more dark-colored 

 minerals, and promptly identifying the rock, regret the intro- 

 duction of the chemical character of the feldspars and the 

 quantitative determination of all the minerals. But to such we 

 have no apology to make. Already precise optical methods of 

 determining the feldspars are in use ; and the confusion result- 

 ing from an absence of the quantitative element in rock defini- 

 tions is becoming intolerable. The time has come when the 

 petrographer should demand the exactness and sharpness of 

 definition which, though obtained only by patient and careful 

 work, add clearness to the conceptions and proportionate weight 

 to the results of petrographic investigation. 



It may be thought that the method of obtaining the approxi- 

 mate chemical composition of the magma by optical study and 

 computation of the minerals in a rock is tedious, as it might pos- 

 sibly take several hours of work. But let one who considers this 

 seriously make a good, careful, and accurate chemical analysis and 

 note the amount of time consumed. We urge him not to throw 

 the task on someone else, especially not on some beginner in ana- 

 lytical chemistry, as has unfortunately so often happened, but to 

 make the analysis himself. Indeed, every petrographer should 

 have made enough analyses to properly appreciate what they 

 mean, after which we feel sure he will not object to the length 

 of time involved in a microscopical analysis when one is possible. 



Again, we have assumed as a condition for proper and con- 

 venient classification that it should be dichotomous, since not 

 more than two factors can be handled advantageously at one 

 time. To those who may not agree with this proposition we can 

 only recommend the trial of forming a classification, using three 



