PETROGRAPHY OF THE KEWEENAWAN 645 



The composition of the mottled type is indicated by the analyses 

 of Table I, but each of the Minnesota samples is considerably altered. 

 Nevertheless the calculated norm cojnfirms the field conclusion that 

 there was considerable variation in the proportion of minerals in 

 the originals, and even some variety in the composition of some of 

 the silicates. 



Hackly diabase. — Microscopically these rocks are usually coarse 

 grained but mineralogically similar to the mottled type. The texture 

 is diabasic as distinct from ophitic, though intermediate textures are 

 not rare. Many of the flows have well-defined amygdaloidal zones. 

 Pumpelly's study of the metasomatic development of secondary min- 

 erals has been checked, as far as material was available, and 

 Minnesota furnishes one type which he did not find well developed — ■ 

 the laumontite rocks. 



Olivine was found in sections from one flow of hackly rock in 

 Minnesota. Pseudomorphs, clearly from olivine, were found in 

 about half the sections of this type; but some rocks, closely resembling 

 these, showed no product or structure indicating that olivine had ever 

 been present. The one Minnesota rock showing fresh olivine is 

 so fresh compared with other flows of the region as to be hard to 

 place with them, but there is no sign of mottling, and the texture is 

 far from the typical ophitic. It gave Analysis 3, of Table II. The 

 olivine is just beginning to alter along the borders and cracks to a 

 chloritic mineral of strong pleochroism in brown and green. A little 

 chlorite appears in other parts of the rock, but no room is left for any 

 glass or its alteration products. This occurrence may well be substi- 

 tuted, in general discussions of the Keweenawan, for the type locality 

 given in Michigan, No, 87 of Marvine's Eagle River section. How- 

 ever, it is not suggested that all hackly diabases were olivinitic. 

 Olivine is present in this freshest sample, and its presence or absence 

 is not a good point to use as a type characteristic. The pseudo- 

 morphs after olivine vary as before, and the development of magnetite 

 and hematite along the cracks gives the pseudomorph the appearance 

 of the high relief of olivine, in fully half the sections examined. Some 

 of the pseudomorphs give a uniaxial figure, as recorded by Pumpelly. 



In the fresh flow of the Snake River series, labradorite is the 

 feldspar, but in the more altered samples very few crystals are fresh 



