102 



GEOLOGY OF THE HIGH PLATEAUS. 



sometimes wanting. The accessory minerals are, with rare exceptions, far 

 inferior to the essential ones in respect to quantity. The following con- 

 spectus exhibits these minerals : 



CONSPECTUS OF JONERALS CHAKACTERISTIC OF THE PRIMARY DIVISIONS OF VOLCANIC ROCKS. 



In addition to the minerals presented in the foregoing scheme, there 

 remain several others of considerable importance. These are chiefly leucite 

 and nephelin. Leucite is found in some basalts replacing the feldspar, and 

 is treated in the classification precisely as if it were plagioclase. Though 

 widely distinct from that group of minerals in its crystallographic forms, it 

 closely approaches them in chemical constitution, differing in this respect 

 mainly in containing a little higher percentage of potash than normal ortho- 

 clase. Nephelin holds exactly the same relations and presents the same 

 distinctions, but holds a high percentage of sod?i instead of potash. It is 

 found not only in the basalts, but also in phonolite, and is generally held 

 to be the most characteristic mineral of the latter rock. If now we treat 

 these two minerals as just so much triclinic feldspar, we shall find no diffi- 



