MICROMETER FOR GEOMETRICAL ROCK ANALYSIS 399 



to increase this minimum to two hundred or even four hundred, 

 and I strongly advocate the increase.) 



2. Two measured lines should be at least the width of a grain 

 apart. 



3. When the constituents are fine-grained and uniformly 

 distributed, measurement of a single section may be sufficient; if 

 coarse-grained, several sections may be necessary in order to satisfy 

 conditions 1 and 2. 



TABLE I 



Total distance traversed, 57.5 mm.; time taken 

 for measurements and calculation, 12 minutes. 



4. In the case of a rock with parallel structure it is necessary, 

 and in most cases it is desirable, to take measurements both along 

 and across the section. 



5. The most accurate method is to measure all the minerals 

 present at the same time, rather than one at a time, although the 

 latter is the quicker way. (This recommendation applies to the 

 use of an ordinary micrometer; it is of course inapplicable to the 

 recording micrometer.) 



6. In the case of coarse-grained rocks it is often quickest to 

 measure a polished face macroscopically, using sections only for 

 minor or microscopic constituents. 



Rosiwal's practice is to draw fine lines on the cover-glass with 

 ink, and then measure along these lines. This is not necessary 



