PRELIMINARY HANDBOOK OF THE DEPARTMENT OF GEOLOGY. 9 



This collection is at present arranged in eight of what are known 

 as standard sloping table cases, and comprises some 1,(500, specimens 

 divided approximately as follows: 



Aqueous rocks 350 specimens; JEolian rocks 20 specimens; Meta- 

 morphic rocks 300 specimens, and Igneous rocks 930 specimens. For 

 further details regarding classification, kinds, and localities represented 

 reference must be made to the handbook and catalogue of this exhibit 

 as it will appear in the Report of the U. S. National Museum for 1890. 



This petrographic collection it has been found advisable to precede 

 by four small exhibits illustrative of (1) structural features, (2) color 

 variations and their causes, (3) specific gravities as influenced by com- 

 position and structure, and (4) the variations in chemical composition. 



It is the object of the first, or structural series, to explain the mean- 

 ing of sundry terms in common use among geologists, bnt whose exact 

 meaning is not always understood by the public at large. Such a col- 

 lection really forms an illustrated glossary, since the meaning of each 

 term, as "porphyritic," "cellular," etc., is shown by means of a specimen 

 in which the structure is the most pronounced characteristic. This col- 

 lection is supplemented by a series of twelve enlarged photomicro- 

 graphs showing the structure of rocks as revealed by the microscope 

 and seen in polarized light. 



In preparing these transparencies, a small chip from each rock was 

 ground so thin as to be transparent (from one five hundredth to one 

 six hundredth of an inch), and then after being mounted between thin 

 slips of glass was photographed through a microscope and between 

 crossed nicol prisms. From the negatives thus prepared further en- 

 largements were made by means of a solar camera, the final print being 

 on glass aud twelve inches in diameter ; that is, that portion of the 

 stone which is in reality about one- fourth of an inch in diameter, is here 

 made to appear one foot in diameter. 



These illustrations were then painted by hand, the artist taking his 

 colors from an examination of the section itself under the microscope. 

 The colors of the various minerals are not, therefore, in all cases the 

 true colors of the minerals themselves, but rather the color they assume 

 when after being cut at different angles with their optic and crystallo- 

 graphic axes they arc viewed by means of polarized light. Such colors 

 are therefore somewhat misleading at first, but are rendered necessary for 

 the purpose of identification and to bring out sharply the lines of sepa- 

 ration between one mineral and another and thus show the structure 

 and composition of the rock. Owing to the thinness of the section it 

 would appear in ordinary light, i. e., light not polarized, nearly color- 

 less or with only dark flecks and faint tinges of color here and there. 



In the second, or color series, an endeavor has been made to arrange 

 the rocks in five groups, showing (1) rocks colored by carbonaceous 

 matter; (2) rocks colored by free oxides of iron ; (3) rocks colored by 

 the prevalence of iron rich silicates ; (I) rocks the color of which is due 



