660 CROSS, IDDINGS, FIRS SON, WASHINGTON 



CLASS II. ORDER 7. RANG I. SUBRANG 3. 



Sal 82.75 7 5 F _ 36-14 ^5^. 3 K,Q'+Na,Q' _ 219 7 K 2 Q' _ 104 5 3 



Fern 15.60^1-^3 L 46.61^-3^5 CaO' o *i Na,0' 115 3 5 



Dosalane. Italare. Lnjavrase. Janeirose. 



The calculation of the norm from, the mode. — Having described 

 the process by which the norm may be calculated from the chemi- 

 cal analysis of a rock, there remains the discussion of the process 

 by which it may be calculated directly from the actual mineral 

 composition of the rock without having the chemical analysis. 



The first requisite in this case is a knowledge of the actual 

 mineral composition of a particular rock, and it is evident that 

 not every rock is sufficiently well crystallized to permit even an 

 approximate estimate of the kinds and quantities of all the min- 

 erals present. Consequently there are very many rocks in 

 which the norm cannot be calculated directly from the rock 

 without recourse to a chemical analysis. These are partly glassy 

 rocks, and those that are so fine-grained that the individual min- 

 eral components cannot be identified and measured. 



But it is possible with some rocks to determine very closely 

 the proportions of the minerals present in them. Such rocks are 

 noncrystalline, and the crystals are sufficiently large to permit 

 their individuality and outline to be recognized. With such 

 rocks the method of determining the quantity of all the mineral 

 components is as follows : 



Estimate by accurate measurement the volumetric propor- 

 tions of all the component minerals. This maybe accomplished 

 by measuring with a micrometer the diameters of each crystal 

 in lines across thin sections of a rock, care being taken to 

 measure a distance at least one hundred times the average grain 

 of the rock. 1 The proportions found for the lengths of diameters 

 of the various components will correspond to those of their 

 volumes. Several other methods have been devised which are 

 less accurate and need not be described here. 



The volumetric proportions are to be reduced to relative 

 masses by multiplying the volume of each mineral by its specific 

 gravity and reducing the total to one hundred parts. 



'Rosiwal, Verh. Wien. Geol. Reichs-Anst., Vol. XXXII, pp. 143 ft., l8 98. 



