BULLETIN 39, UNITED STATES NATIONAL MUSEUM. [8] 



For carrying specimens, a stout leather bag, made Avitli separate 

 compartments for compass and notebooks, affords the only feasible 

 means (fig. 9). This can be carried by means of a single strap in the form 

 of a loop over the shoulder or with two straps, like a knapsack. The 

 former is most easily put off and on, and hence has its advantages. 

 The knapsack form can, however, be carried with much less fatigue. 

 One needs to be careful and not let his enthusiasm cause him to get 

 too large a knapsack, or to attempt carrying too much at one time. The 

 remarkable rapidity of increase in weight of a satchel of rocks can be 

 appreciated only by one who has tramped the weary miles homeward 

 after a hard day's field work. 



PART II. — THE PREPARATION OF THIN SECTIONS. 



Since the study of rocks is now carried on so largely by means of 

 thin sections and the microscope, a few words regarding the methods 

 of preparation of the sections may not be out of place here. 



The purpose of the section is to get a certain portion of the rock so 

 thin as to be transparent, without disturbing its structure or the loss 

 of any of its particles. The process of preparation, as now generally 

 employed, has come into general use only within the past twelve or 

 fifteen years, the first monographic work of importance to appear in 

 this country being Professor Zirkel's Microscopical Petrography, 

 which formed Yolume vi of the Monographs of the United States 

 Geological Survey of the Fortieth Parallel under Mr. Clarence King, 

 and was published in 1876.^ 



The efficacy of the method is based upon the fact that every crystal- 

 lized mineral has certain definite optical properties, that is, when cut in 

 such a way as to allow the light to pass through it, will act upon that 

 light in such a way as to enable one working with an instrument com- 

 bining the properties of a microscope and stauroscope to ascertain at 

 least to what crystalline system it belongs, and, in most cases, by study- 

 ing crystal outlines and lines of cleavage, the mineral species as well. 

 To enter upon a detailed descrijDtion of the methods by which this is 

 done would be out of place here, since it involves the polarization of 

 light and other subjects which must be studied elsewhere. The reader 



'Among the earlier American workers were Dr. G. W. Hawes, who, in 1878, pub- 

 lished an important work entitled Mineralogy and Lithology of New Hampshire, 

 forming Part IV (251 pp. and 12 colored plates) of Volume III, Geology of New Hamp- 

 shire, by C. H. Hitchcock; Prof. R. Pumpelly, of Newport, R. I.; Dr. M. E. AVads- 

 worth, of Cambridge, Mass. ; J. H. Caswell, who first described the phonolites of 

 the Black Hills (Geology of the Black Hills of Dakota, 1880); Dr. A. A. Julien, of 

 'New York; and Prof. B. J. Harrington, of Montreal. The first systematic attempt 

 at section cutting in the United States was made by Professor Julien, at Columbia 

 College, in New York. In the winter of 1881 apparatus was set in motion for this 

 purpose in the National Musuem, under direction of Dr. G. "W. Hawes, and later 

 (1884) by the United States Geological Survey in the Hooe Building, in the same 

 city. See Modern Petrography, by Dr. George H. Williams, D. C. Heath & Co., 

 Boston, 1886, for a brief history of this branch of the science. 



