10 REPORT OF NATIONAL MUSEUM, 1889. 



in part at least to structural features and the transparency of feld- 

 spatbic constituents, and (5) rocks the color of which is also in part due 

 to the physical condition of the various constituents, but more particu- 

 larly to a lack of carbonaceous matter, iron or other metallic oxides. To 

 this series is appended another showing the changes in color due (1) to 

 the bleaching of the carbonaceous matter; (2) to the leachiug out of fer- 

 ruginous oxides by organic acids ; (3) to the oxidation of iron protoxide 

 carbonates or sulphides ; (4) to a like change in the iron rich silicates, 

 and (5) to a change in the physical condition of the constituent min- 

 erals, mainly the feldspars. 



It is not necessary to here enlarge upon the third and fourth of the 

 small series mentioned above, further than to say that the difference 

 between the apparent and real specific gravity is rather strikingly 

 shown in three contiguous glass jars each partially filled with water. 

 The first contains a piece of pumiceous obsidian, which, buoyed up by 

 its numerous vesicles, floats readily at the surface ; the second contains 

 the same pumice, but sufficiently pulverized to admit the water into its 

 vesicles. The fragments, therefore, sink to the bottom, as does the 

 compact nonvesicular portion of the same obsidian in glass No. 3. This 

 series is described in detail in the handbook above referred to. 



Aside from the collections described above as forming the exhibition 

 series, as illustrative of the mineral aggregates forming any appreciable 

 proportions of the earth's crust, there are in the department, stored 

 away in the table cases, many collections, designed primarily for study 

 only. These are so arranged as to be accessible to the student on ap- 

 plication to the Director of the Museum and on presentation of proper 

 credentials, if such be deemed necessary. The collections thus stored 

 are classed under the head of the study series. In preparing and 

 arranging this series it may be well to state that it is made up largely 

 of such materials as have somewhere and at some time been subject 

 to investigation. Each specimen, after trimming to a size approxi- 

 mating 3 by 4 by 1 inch, has a number painted on it in oil colors, and 

 which refers to a written catalogue, in which is given whatever de- 

 tailed information regarding its source aud nature may be in posses- 

 sion of the department. They are then placed in pasteboard trays, 

 accompanied by written labels containing the same information as 

 given in the catalogue, and placed in the drawers of the table cases. 

 Material which is designed for the study series is, if of a miscellaneous 

 nature, distributed through the collections in a systematic manner, cor- 

 responding to that adopted for the exhibition series. Collections which, 

 like those from Leadville and the Eureka District, to be noted later, 

 represent systematic work upon rocks of a definite area, or which have 

 been studied as a group for the elucidation of some particular problem, 

 are kept intact, in order to best serve the purposes of the investigator. 

 Characteristic rocks have in some cases been selected from these col- 

 lections for exhibition purposes, but the individuality of the collection 



