DALY. — COMPOSITIONS OF IGNEOUS-ROCK TYPES. 



235 



Tinguaite 92 



Trachydolerite 60 



Trachyte 14 



Urtite 22 



Vogesite 95 



Websterite 52 



Wehrlite 53 



Average Specific Gravities of Certain Types. 



The average specific gravities of holocrystalline types have been 

 calculated, with result shown in the following accessory table. Most 

 of the determinations were taken from Osann's book. 



Some Applications. 



The uses to which the averages may be put are diverse and, in cer- 

 tain instances, direct and important. A brief note in this place will 

 indicate something of the range of the considerations affected. 



1. The writer has found from personal experience that the averages 

 have been of decided benefit in showing the chemical individuality and 

 true nature of the igneous-rock types as actually mapped. To student 

 and investigator alike such averages are, for many purposes, more 

 valuable than single analyses. They help to show that eruptive rocks 



