1887] Mineralogy and Petrography. IILI 
silicates. (4) The stronger tendency of sodium compounds to 
separate, as compared with potassium compounds, is seen in the 
case of the sodium-bearing hornblendes and augites. (5) Sani- 
dine crystallizes from a molten magma only after the relative 
proportions (molecular) of K,O and Na,O in it become as 2:1. 
(6) The affinity of the elements for each other is an important 
factor in determining the order of crystallization. Calcium and 
sodium are widely found associated in the same minerals, mag- 
nesium and sodium rarely. Potassium, on the other hand, occurs 
frequently with magnesium, rarely with calcium, etc. (7 e 
of the different portions of a rock, Lagorio also discusses several 
theoretical questions of general interest. He believes that the 
hornblende and mica in rocks were formed in the presence of 
water? and under pressure. The existence of basic hydrogen in 
many members of the mica group, he thinks, would indicate this 
The rocks of the range he divides into three classes,—eruptive, 
foliated, and stratified. The first consists principally of Pree ed 
_ Glabases, gabbros and granites, and felsites. These he 
_* Cf. American Naturalist, 1886, Pp» 160. 
o TID., February, 1887, p. 172. 
_ VOL, XXI,—No, 12. 75 
3 August, 1887, p- 481- 
