PBTKOGRAPHIC GEOLOGY AND DESCRIPTIONS. 



117 



Diabase.] 



thus changed show their nature sometimes by the remnants left of the original 

 grains, and nearly always by their relation to the feldspars, which they preceded in 

 origination. Although the changed olivine resembles somewhat the product of 

 change from augite, these two are frequently alongside of each other without inter- 

 penetration. The figure below (figure 5), illustrates the different aspects of the 

 augite and the olivine. It was drawn from No. 15 by camera lucida and shows the 

 parts all magnified about seventy-five diameters. The changed mineral (olivine) is 

 frequently closely associated with the unchanged (augite) in so much that in some 

 instances, either by superposition or by some corresponding alteration in the augite, 

 there is difficulty in separating them at a definite boundary. They both enclose 

 irregular grains of magnetite. The olivines, however, crowd upon and indent the 

 feldspars, though their forms are never perfect, while the augites, being later than 

 both, are interrupted by their contours. 



FIG. 5. 



No. 1-Olivine. 

 No. 2 Augite. 



No. 3 Labradorite. 



No. 4 Magnetite. 



Magnetite is in all forms, especially in angular to subangular sections, sometimes 

 almost square, and in rods. These rods are sometimes between two minerals, or 

 between cleavages, but they are also often sections of tabular sheets of magnetite. 

 The largest amount of magnetite is within the decayed olivines. 



One section examined. 



Af/c. Manitou (?) 



Remarks. This rock is noticeably contrasted with the eruptives which it cuts, 

 in two respects: (1) It is not porphyritic nor amygdaloidal, but has a uniform 

 granular structure. (2) It is comparatively fresh, with a preserved ophitic structure. 



N. H. W. 



