204 EEPOET OF NATIONAL MUSEUM, 1888 



excluded, as belonging more properly to the department of palaeontology. 

 As at present contemplated, the main idea in view is to show that the 

 same geological forces have been in operation, and that rocks of the same 

 general nature have been in process of formation from the earliest 

 times down to the most recent. This collection, although now compris- 

 ing some four hundred specimens, is far from complete, and can not be 

 satisfactorily arranged for lack of proper cases. 



The following show the character of labels used in the various col- 

 lections. All are exact copies of labels now in use. 



[form of LA.BEL USED IN COLLECTION OF ROCK-FORMING MINERALS.] 



ORTHOCLASE. Potash feldspar. 



Composition: K2Al2Si60i6 = silica 64.7 per cent; 

 alumina 18.4 per cent; potash 16.9 per cent 



Crystalline System: Monoclinic. 



An essential constituent of granite, gneiss, 

 syenite, and orthoclase-porphyry ; occasionally 

 found in more basic eruptive rocks, such as dia- 

 base. Under the name sanidin is included the 

 clear, glassy variety of orthoclase occurring in 

 Tertiary and modern lavas, as trachyte, phonolite, 

 and liparite. Orthoclase often occurs in large 

 deposits, and is quarried and used in the manu- 

 facture of pottery. 



[Form of label used in structural series of rocks.] 



Vitreous Rocks. 



VESICULAR STRUCTURE. 



Obsidian Pumice. 



Mono Lake, California. 29,63^0 



Collected by G. K. Gilbert, 1883. 



[Form of label used iu general rock collection. ] 



AUGITE SYENITE. 



Tin Mines, Jackson, New Hampshire. 29,587. 



Gift of Dr. G. W. Hawes. 



