202 REPORT OF NATIONAL MUSEUM, 1888. 



ueed be considered an obstacle, since a glass jar of hydrogen, though 

 apparently empty, is, if properly labeled, as instructive as though the 

 substance itself were tangible. Samples of the various forms of labels 

 employed, in this as well as the other collections, will be given later. 

 In the collection of rock-forming minerals the specimens are selected 

 not for their beauty or fine crystallographic development, but ordinary 

 forms, both crystalline and massive, are shown in all their principal 

 varieties. The collection is divided into (1) primary minerals, or those 

 which formed at the time of the consolidation of the rock, and (2) sec- 

 ondary minerals, or those which have formed since its consolidation, 

 and are due mainly to decomposition, hydration, or solution and recrys- 

 tallization. The individual labels further state whether the mineral is 

 an essential or accessory constituent and of what class of rocks it forms 

 a part. In the rock collection the samples are as a rule trimmed with 

 a hammer into sizes approximately 3J by 4J by 1 inch, this size having 

 been found most convenient when everything is taken into considera- 

 tion. The rule is not, however, inviolable, and both size and shape are 

 allowed to vary when the character of the rock renders this advisable. 

 Care is taken in all cases to procure fresh and characteristic material 

 and that no specimen shall show abrasive marks from the hammer or 

 other agencies on its exposed surface. This collection it has been found 

 advisable to precede by a struciural series, *. e., a small series showing 

 all the common forms of rock structure. It is the object of the collec- 

 tion to explain the meaning of sundry terms in common use among 

 geologists, but whose exact meaning is not always understood by the 

 public at large. Such a collection really forms an illustrated glossary, 

 since the meaning of each term, as i)orphyritic, cellular, etc., is shown 

 by means of a specimen in which this structure is the most pronounced 

 characteristic. This collection is supplemented by a series of twelve 

 enlarged photo micrographs, showing the structure of rocks as revealed 

 by the microscope and seen in polarized light. The general rock col- 

 lection I should say is classified by kinds, regardless of locality or 

 economic value. The rocks of strictly economic value, exclusive of ores, 

 are now grouped by themselves in the extensive collections of (a) build- 

 ing and ornamental stone and (b) stones used for abrading purposes. 

 The first-named of these, as I have stated in previous reports, it has 

 been found advisable to have cut in the form of 4-inch cubes, polished 

 on the front face when the nature of the stone permitted. The remain- 

 ing faces being finished as follows : Drafted and pointed on the left 

 side; drafted rock face on the right side, rock face behind, and smooth 

 sanded or chiseled on the top and bottom. In special cases, as with 

 coarse figured or remarkably unique marbles, it has been found advis- 

 able to deviate from this rule and finish the stone in the form of a thin 

 ■slab from 4 to 12 or even 24 inches in greatest dimensions. Finished 

 objects of stone, excepting so far as they demonstrate the adaptability 

 of a rock to some particular purpose, are as a rule excluded as belong- 



