80 EOCKS FOEMED THEOUGH IGNEOUS AGENCIES 



porphyritie feldspars and small scales of mica, hornblende, or 

 whatever may be the prevailing accessory; pumiceous forms 

 are not nncommon. Under the microscope the ground-mass is 

 found to vary from clear glassy through microlitic forms to 

 almost holocrystalline. The minerals of the ground-mass are 

 feldspars in elongated microlites, specks of iron ore, apatite in 

 very perfect forms, and one or more of the accessory ferro-mag- 

 nesian minerals. 



Colors. — The prevailing colors are some shade of gray, green- 

 ish or reddish. 



Classification and Nomenclature. — Specific names are given 

 dependent upon the character of the prevailing accessory. We 

 thus have: Quartz andesites or dacites; Hornblende andesites; 

 Augite andesites; Hypersthene andesites; and Mica andesites. 



The glassy varieties are often known as hyaline andesites. 

 The name propylite was given by Eichtofen to a group of 

 andesitic rocks prevalent in Hungary, Transylvania, and the 

 western United States but the rocks have since been shown by 

 Dr. Wadsworth^ and others to be but altered andesites, and the 

 name has fallen largely into disuse. 



5. THE aABBRO-BASALT GBOtTP 



This is a large and variable group of rocks which on struc- 

 tural and mineralogical grounds might well be subdivided. Thus 

 the gabbros, norites, and hypersthene andesites might well be 

 considered as a group by themselves, while the diabases, augite 

 porphyrites, melaphyrs, and basalts would form a second. Ow- 

 ing, however, to the similarity of the magmas from which they 

 have been derived, it is believed the wants of the student will be 

 best subserved by grouping them all together as above. They 

 may be described in detail as below : 



(1) THE GABBEOS 



Gabbro, an old Italian name originally applied to serpen- 

 tinous rocks containing diallage. 



Mineral Composition. — The gabbros consist essentially of a 

 basic soda-lime feldspar, either labradorite, bytownite, or an- 

 orthite, and diallage or a closely related monoclinic pyroxene, 

 a rhombic pyroxene (enstatite or hypersthene), and more rarely 



^Proc. Boston Society of Natural History, Yol. XXI, 1881, p. 260. 



