520 REPOET OF NATIONAL MUSEUM, 1890. 



II. — SPECIFIC GRAVITY. 



The term specific gravity is used to designate the weight of any sub- 

 stance when compared with an equal volume of distilled water at a 

 temperature of 4° G. This property is therefore dependent upon the 

 specific gravity of its various constituents and their relative proportions. 

 The exact or true specific gravity of a rock may be obscured by its 

 structure. Thus an obsidian pumice will float upon water, buoyed up 

 by the air contained in its innumerable vesicles, while a compact obsi- 

 dian of precisely the same chemical composition will sink almost in- 

 stantly. This property of any subject is spoken of as its apparent 

 specific gravity in distinction from the actual comparative weight bulk 

 for bulk of its constituents parts, which could in the case of a pumice 

 be obtained only by finely pulverizing so as to admit the water into all 

 its pores. This difference between the apparent and true specific grav- 

 ities is illustrated in the three specimens of obsidian pumice, pulverized 

 obsidian pumice, and compact obsidian Nos. 39348 and 39348a. The 

 first, it will be observed, floats readily owing to the buoyancy of the air 

 included in its vesicles ; the second glass contains the same rock pul- 

 verized so that all the air-chambers are broken open and the air escapes. 

 The rock fragments consequently sink to the bottom , as does the third 

 of the series (No. 29631), a compact, nonvesicular mass of the same ob- 

 sidian. Inasmuch as the structural peculiarities of any igneous rock — 

 as will be noted later — are dependent upon the condition under which 

 it cooled, it is instructive to notice that the crystalline aggregates have 

 a higher specific gravity, i. e. a greater weight, bulk for bulk, than does 

 a glassy, noncrystalline rock of the same chemical composition. The 

 property is therefore dependent upon chemical (and consequently min- 

 eral) composition and structure, and as a very general rule it may be 

 said that among the siliceous rocks those which contain the largest 

 amount of silica are the lightest, while those with a comparatively small 

 amount, but are correspondingly rich in iron, lime, and magnesian con- 

 stituents, are proportionately heavy. 



III. — THE CHEMICAL COMPOSITION OF ROCKS. 



This varies naturally with their mineral composition. It is customary 

 to speak of rocks as calcareous, siliceous, ferruginous, or argillaceous, 

 accordingly as lime, silica, iron oxides or clayey matter are prominent 

 Constituents. Among eruptive rocks it is customary to speak of those 

 showing on analysis upwards of 60 per cent, silica as acidic and those 

 showing less than 50 per cent., but rich in iron, lime, and magnesian 

 constituents, as basic. The extremes as will be noted are represented 

 by the rocks of the granite and peridotite groups. 



The series illustrating the above-mentioned properties are arranged 

 as below. With the eruptive rocks only the silica percentages are 

 here given. The results of the complete chemical analysis of each 

 variety are given further on, in the pages devoted to their description. 



