MICROMETER FOR GEOMETRICAL ROCK ANALYSIS 395 



3. Methods 1 and 2 can be applied to microscopic sections by 

 the aid of a camera lucida attached to the microscope. An ordinary 

 photographic camera can also be used, the outlines of the grains 

 being sketched from the enlarged image on the focusing screen. 



4. If a dark room is at hand, it is sometimes preferable to photo- 

 graph a rock section instead of sketching it. The print can be 

 examined either by weighing or by means of squared paper. 



5. A squared ocular micrometer, by means of which the areas 

 of the grains in a section can be measured directly under the 

 microscope, was tried by Rosiwal. It was found to be less advan- 

 tageous than the following method, viz. : 



6. Rosiwal's linear traversing method: 1 If the work be executed 

 with care and under all necessary precautions, this is the simplest 

 and perhaps for that reason the most accurate of all geometrical 

 methods of rock analysis. The measurement of areas is replaced 

 by the measurement of diameters along a selected line or lines. 

 Either a microscopic section or the smooth face of a hand-specimen 

 of the rock may be employed, according to whether the rock is of 

 fine or coarse grain. In the latter case, a graduated rule or tape 

 is required; in the former, an ocular micrometer. Any kind of 

 micrometer will do, but the estimation is facilitated by the use 

 of certain special types, such as the "planimeter ocular" of Hirsch- 

 wald. 2 Subject to certain conditions, the number representing 

 the sum of the diameters of all grains of one kind is proportional 

 to the volume of the mineral concerned. 



So far as tediousness is concerned, all these methods are more 

 or less on the same level; the measurements are very wearisome 

 and take a long time to perform. Generally speaking, one would 

 expect weighing to be a more exact process than the use of squared 

 paper, but then the weighing must be preceded by sketching and 

 cutting out, and appreciable errors may creep in during these 

 manipulations; furthermore, one cannot be sure that the material 

 weighed, be it tinfoil, cardboard, or paper, is everywhere of the 

 same thickness. On the other hand, the counting of innumerable 



1 Rosiwal, "Uber geometrische Gesteinsanalysen," Verhandlungen der k.k. geolog. 

 Reichsanstalt, Wien (1898), No. 5. 



2 J. Hirschwald, Centralblatt fur Min., Geol., Pal. (1904), No. 20. 



