REPORT OF NATIONAL MUSEUM, 1889. 



SECTION A.— SYSTEMATIC GEOLOGY, 



I. — GEOGNOSY: A CONSIDERATION OF THE MATERIALS OF THE 



EARTH'S SUBSTANCE. 



Under this section are shown : (1) the sixteen more common elements 

 which, in their various combinations, are estimated to form about 99 

 per cent, of the substance of the eartlrs crust; (2) the minerals 

 which, resulting from these combinations, go to make up rock masses; 

 and (3) the rocks themselves. All the substances required for these ex- 

 hibits are of such nature as to admit of their being readily utilized. Not 

 even the fact that an element is a gas, both colorless and tasteless, is 

 to be considered an obstacle, since a glass jar of hydrogen, though ap- 

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

 substance itself were tangible. In the collection of rock-forming 

 minerals, the specimens are selected not for beauty or fine crystallo- 

 graphic 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 consol- 

 idation of the rock, and (2) secondary minerals, or those which have 

 formed since its consolidation, and are due mainly to decomposition, 

 hydration or solution and re-crystallization. The individual labels 

 further state whether 1 he mineral is an essential or accessory constituent, 

 and of what class of rocks it forms a part. 



The following shows the form of label here used : 



APATITE. 



Compositiott: Phosphate of lime. 

 Crystalline System: Hexagonal. 



Common as an original constituent in granular 

 limestone, syenite, gneiss, mica, and hornblendic 

 schists and occasionally found in serpentine. Oc- 

 curs also in slender, colorless, usually microscopic 

 crystals in many igneous rocks oi all ages, such as 

 granite, basalt, andesite, diorite, and diabase. 



