38 



Evolution and Distribution of Fishes 



In geological treatises such deposits have been referred 

 to and in some cases described at considerable length. 

 But their importance as possible conserving agents from 

 the palaeontological standpoint seems almost wholly to have 

 been overlooked. A somewhat detailed consideration of 

 them seems therefore to be appropriate here. 



As to the nature of the volcanic dust, this varies greatly 

 according to locality, and the composition of the crustal 

 rock that has undergone pulverizing. But as granitic masses 

 constitute a large portion of the foundational crust of the 

 earth these must most commonly have been reduced to 

 powder and exploded from vents. The following table, 

 therefore, is arranged to show the chemical composition 

 of a series of granitic rocks. 



The above shows that silica, alumina, and iron oxide 



make up about 90 p.c. of the total granitic mass. It is 



Constituents No. i No. 2 



Silica (Si02) 7i-i5 68.91 



Alumina (AI2O3) and Iron (Fe203) i5-95 6.12 



Lime (CaO) 0.85 3.44 



Magnesia ( MgO ) 0.41 .... 



Manganese (MnO') trace .... 



Potash (K2O) 3.36 0.36 



Soda (Na20) 4.94 3.09 



Organic Matter 8.75 



Sulphuric Acid (SO3) 8.88 



of interest therefore to compare with this the analysis of 

 two samples of volcanic dust as given by 1. C. Russell in 

 his valuable and original work (77:292), No. i being 

 from Truckee Canyon, Nevada, and No. 2 from Nebraska. 



