MICROMETER FOR GEOMETRICAL ROCK ANALYSIS 403' 



It would be well to allow a greater travel to sledge A by slightly 

 increasing the length of sledge B — say to 112 mm. 



A well-finished instrument of this kind could not be made for 

 much less than £5, hence it is not likely to become part <3f every 

 petrologist's equipment; but in view of the saving of time and 

 eyesight which it effects the initial outlay is inconsiderable, and 

 the gain to descriptive petrography, if it should succeed in 

 popularizing the geometrical analysis of rocks, would be very 

 great. 



The instrument has been examined by Messrs. J. Swift & Son, 

 London, who have all the information necessary for executing 

 copies of it. 



EXPRESSION OF THE " COLOR RATIO " OF A ROCK 



It has always appeared to me to be a matter of great importance 

 in rock descriptions to state the proportion of light to dark minerals 

 accurately. The terms " leucocratic " and " melanocratic " have 

 proved extremely useful in giving a rough indication of this pro- 

 portion (which I am in the habit of calling the "color ratio" of 

 the rock), but something more is urgently required. Of course 

 the fundamental point of difference between the light and the 

 dark minerals does not lie in their color, but in their specific gravity, 

 to which, however, the color affords a convenient index, inasmuch 

 as all minerals of gravity less than 2 . 8 are leucocratic (predomi- 

 nantly light-colored) and those of higher gravity are melanocratic 

 (predominantly dark-colored). It is becoming more and more 

 apparent that differences of specific gravity must be reckoned 

 among the chief causes of magmatic differentiation, and for this 

 reason, as well as for its purely descriptive value, the color ratio 

 must receive quantitative recognition in the future. 



With the micrometer described above, it is possible to measure 

 the color ratio of a fine-grained rock with a high degree of accuracy 

 in a period of ten to thirty minutes, according to circumstances. 

 Having ascertained the ratio, the next question is how best to 

 express it, and the way which does least violence to our accepted, 

 illogical system of nomenclature is to resort to a system of prefixes. 



