PETilOGRAPHIC GEOLOGY AND DESCRIPTIONS. 429 



Diabase.] 



In a section whose thickness is less than 0.03 millimeter the double refraction 

 shows a coloration only when the fibres are cut perpendicularly, and then a straw 

 yellow color. The actual double refraction is therefore somewhat less than 0.015. 



Five sections. 



Age. Cabotian(?) 



Remark. In the trap rock at Rock Harbor every transition can be seen between 

 the amorphous green mineral (" lintonite "?) to thomsonite and to chlorastrolite. 

 A variety of pebbles gathered on the beach (No. 570 B) illustrates this. In some of 

 the amygdaloidal cavities prehnite also seems to have been formed, and it is 

 associated with metallic copper, as noted at French river (rock No. 80). N. H. w. 



No. 571. DIABASE (with olivinej. 



Prom the Siskiwit mine, Isle Royale, near Rock harbor. 

 Ref. Annual Report, x, page 53. 



The feldspar is specked and even crowded with zoisite, but its albite twinning 

 is very well preserved. Extinction on n p is 43, indicating labrador-bytownite. 



Auyite is almost lost by decay which has entered along the cleavages, leaving 

 a grouping of isolated grains which extinguish in unison, having an ophitic relation 

 to the feldspars. 



Olivine has changed into two products which occupy the places of the original 

 grains, viz.: 



1. A brown opaque substance which resembles boivlinyite. 



2. A translucent but nearly isotropic substance. 



In natural light the former often forms ferruginous veins which penetrate the 

 latter, and by encroaching on it in greater and greater amount either occupies the 

 whole of the space of the original olivine, or leaves only a nucleus which remains 

 translucent. The translucent mineral, in natural light, has sometimes about the 

 color and forms of the original olivine. 



This nearly isotropic product of alteration of olivine has been met with 

 frequently in the traps of the state, in the course of this examination, and it has 

 frequently been called serpentine. But, as Prof. Lacroix shows (Mineralogie de 

 France etde ses Colonies, page 417), it is a name without definite mineral significance, 

 and had better be reserved for the rock in which these products constitute the mass, 

 and the products can be assigned to more definite mineral substances. In this case 

 the substance is hardly isotropic but seems in some cases to have a vague, coarsely 

 felted extinction which approaches to a parallel fibrous extinction, and in others its 

 fibrous structure is more evident, and it then resembles thalite. 



Quartz as a secondary product is in grains of considerable size, embracing other 

 minerals poikilitically. It is not common. 



