954 THE GEOLOGY OF MINNESOTA. 



[Augite. 



cases the causes of these alterations, inherent in the accidental environment, are 

 easy to see. The chief of these is plainly solfataric activity, penetrating and 

 attacking the lava during the cooling period. Its purveyors were hot solutions and 

 gases, the chief of the latter being steam. Secondly should be mentioned endomor- 

 phism from the contacts on the surrounding acid rocks. These agents were cotem- 

 porary and complementary. They had their greatest effect where the augites are 

 wholly altered to hornblende (Nos. 554, 1848, 1849) and where the plagioclases are 

 reddened. This is a combination which is sometimes accompanied by widespread 

 interfusion and mingling of the basic magma with extensive areas of the acid, the 

 two becoming completely and mutually mingled so as to produce igneous rocks of 

 intermediate type in the Keweenawan (Nos. 5, 648, 650, 674, 675). 



There are also some petrographical peculiarities appertaining to augite exam- 

 ined in the Keweenawan and in the muscovadytes: 



(1) Its optic angle is sometimes very small, being apparently not over 5, but 

 varying to 45 (Nos. 126, 223, 291, 297, 1828, 2001, 178E), and it has been noted that 

 this is accompanied in the same rock by the ophitic relation of olivine to plagioclase 

 (No. 1828). 



(2) It appears sometimes to have two generations in the same rock, i. e., it 

 is both granular and earlier than, or cotemporary with, the generation of the 

 plagioclases, and is ophitic in its relation to the plagioclases (Nos. 89, 133, 222, -L'S. 

 229, 515, 517, 615, 820, 2064). In other cases it is wholly granular (No. 122) or 

 wholly ophitic (Nos. 53, 106, 108, 625). The ophitic structure is uniformly the latest 

 to form, at least it is later than the granular when both exist in the same rock (Nos. 

 515 and 517). The granular condition prevails in the muscovadytes and in the gabbros 

 proper, but it is not confined to the coarser grained rocks. It appears in the finest 

 of the diabases (No. 654), even in zirkelyte or glassy diabase (Nos. 540, 541). In 

 No. 547 augite is porphyritic in a glassy base. In the muscovadyte it graduates 

 apparently into the "globular structure" (Nos. 1334, 1347) described below. It is 

 plain that the granular structure cannot, of itself, be said to be characteristic of 

 gabbro, nor the ophitic of diabase. On the other hand, this intimate association of 

 the granular and the ophitic structures in the same rock destroys the usefulness of 

 this distinction as a character on which to base nomenclature, and points to the con- 

 clusion that diabase and gabbro cannot be dissociated on a genetic basis. 



(3) It has been customary to account for these two structures of augite on the 

 assumption that the granular was formed under a state of flow in the rock when the 

 crystallizing points were continually separated, and the ophitic after the rock came to 

 rest. There are, however, some facts that can hardly be explained in that way. (a) 

 The ophitic structure prevails in the surface lavas and dike rocks, which are admit- 



