232 REPORT OF NATIONAL MUSEUM, 1884 



strata, which will be of great use in preparing the collection for exhi- 

 bition. 



The collections of the department, as at present arranged, are broadly 

 generalized under the heads of lithology — including the building and 

 ornamental stones — and physical geology. 



Lithological collection. — The lithological collection proper comprises 

 all varieties of rocks of scientific or educational value, without regard 

 to their economic importance. Bocks belonging to this series are, for 

 convenience sake both in handling and storing, broken into blocks 

 about 4 by 3 by 1 inches, with the edges carefully trimmed and the 

 faces showing fresh fractures, with no abrasion marks from the ham- 

 mer. The catalogue number is then painted on each, and they are 

 stored in the drawers of the table-cases for purpose of reference and 

 study, or placed upon exhibition, as the case may be. Each speci- 

 men thus prepared is accompanied with a written or printed label giv- 

 ing what information is known in regard to the same or referring to 

 sources from which other information can be obtained. In arranging 

 the rocks of this series, either for exhibition or for reference and study, 

 collections illustrating the formations of any definite area, or investiga- 

 tions tending toward the solution of any particular problem, are pre- 

 served intact, while miscellaneous collections are broken up, classified 

 by kind, and distributed throughout the general collections of the de- 

 partment. 



The exhibition series of this collection comprises now some 800 speci- 

 mens, but this number can easily be doubled as soon as proper cases 

 are prepared. As a supplement to this collection, to explain their 

 structure and mineral composition, a series of enlarged photomicrographs 

 of twelve thin sections of typical rocks have been prepared. These en- 

 largements are in the form of transparencies, 12 inches in diameter, and 

 are colored by hand, the artist taking his tints from an examination of 

 the sections themselves under the microscope and in polarized light. 

 The illustrations thus prepared are very accurate as well as attractive, 

 and cannot fail to add greatly to the value of the educational series of 

 this department. 



With particular reference to educational purposes, two special col- 

 lections have been prepared during the past season ; one a structural 

 series of rocks, and the other a series of rock-forming minerals. The 

 structural series is desigued to show all the more typical forms of rock 

 structure and texture; in other words, to illustrate by means of speci- 

 mens the meanings of certain words and phrases in constant use in 

 lithological nomenclature, but whose exact significance or force is poorly 

 comprehended by the public in general. In this collection the rocks are 

 divided primarily into three groups: {a) crystalline, (&), vitreous, or 

 glassy, and (c) clastic, or fragmental, under which are arranged all those 

 forms of structure common to each. The collection thus includes three 

 nearly parallel series, and comprises, as at present arranged, forty-one 



