BASALTIC GEOUP. 



Ill 



an intermediate variety of texture (anamesite) has been named in which the 

 crystallization is recognizable but not conspicuous. I fail to discover suffi- 

 cient reasons for a subdivision on textural characters alone, but differences 

 of habitude which are tolerably constant may, I think, be founded upon 

 the mineralogical constitution. The basalts almost invariably contain 

 olivin in abundance, while in the dolerites it is far less common though 

 sometimes found. The dolerites are as a group more siliceous, though the 

 true basalts sometimes have more than the normal percentage of that consti- 

 tuent. In the true basalts such minerals as augite, magnetite, olivin, leucite, 

 and nephelin reach the extremes of their proportions ; in the dolerites the 

 same minerals are on the whole less abundant, and the predominance of the 

 feldspathic ingredient is more emphatic. It has seemed to me, therefore, 

 that the name dolerite should be fully recognized as applicable to a sub- 

 gi'oup of the basalts, including those coarser-grained varieties in which the 

 proportion of silica is notably higher than in the typical basalts, and also 

 including the more basic of those rocks which Zirkel has called augitic 



andesites. 



Group IV.— BASIC EOCKS— BASALTS. 



Sub-groups. 



Characteristics. 



1. Dolerite 



Distinctly crystalline ; plagioclase feldspar 'with (usually) subordinate 

 orthoclase; augite always conspicuous and in large amount ; much 

 magnetite ; a glassy base with pronounced fluidal texture ; formless 

 clots of black ferruginous material usually considered as amorphous 

 augite. Color, dark gray to nearly black. 





2. Nephelin-dolerite 



Similar to the above but with nephelin replacing a part of the plagio- 

 clase. 





Fine-grained ; feldspar crystals distinguishable only by tho microscope. 

 Abundant augite and a glassy base ; olivin usually present. Very 

 dark colored, nearly black. 





4. Leucite-basalt 



With leucite replacing a part of tho feldspar and sometimes the whole 

 of it. 





With nephelin replacing feldspar. 



6. Tachylite 



A vitreous obsidian-like lava, having tho basaltic constitution. 







