176 THE GEOLOGY OF MINNESOTA. 



[Diabase. 



Mey. A rather coarse, irregular and spotted rock of the diabase order. The 

 spots are due apparently to segregations of quartz in the form of chalcedony, and 

 perhaps to thalite disseminated through the mass of the rock. The rock is also 

 spotted with darker areas, apparently due to aggregations of the pyroxene elements 

 and to the formation of chlorite. The metalloidal reflections sometimes spread 

 over adjoining parts of a large crystal which embraces the other minerals in a 

 poikilitic manner. 



Mic. It is similar in its essential characters to several other diabases already 

 described. The feldspar is embraced by the pyroxene, and appears in twinned lath- 

 shaped grains, clouded with chlorite and other inclusions, and twinned on the albite 

 plan abundantly, and rarely on the Baveno plan. A grain cut parallel to the 

 brachypinacoid, gives an axis of elasticity oblique to the plane of the section, the 

 interference figure consisting of a curved black bar which crosses the field, with an 

 extinction angle on the basal cleavage of 28, indicating labradorite or labradorite- 

 bytownite. 



The pyroxene has sometimes a conspicuous close cleavage, but in general it 

 presents the usual characters of auyite, and is quite fresh. It has an ophitic relation 

 to the feldspar crystals. The metalloidal reflections, apparently parallel to 010, do 

 not appear in thin section. 



Olivine is not abundant, and has been changed to serpentine, presenting a 

 yellowish color in ordinary light, with a fibrous or felted structure. 



Magnetite has apparently the angular form of some mineral which was posterior 

 to the feldspars; perhaps the position of the remnants of the non-differentiated 

 magma. Its partial change to leucoxene shows that it is titaniferous. 



Thomson ite appears, both macroscopically and microscopically. It is white, 

 finely felted and in formless secondary masses. The periphery of these geodic nests 

 consists occasionally of a coarser radiation of the same mineral, although it might 

 be taken, at a casual glance, for quartz. In some cases it is dirty green, but 

 frequently differs from the central portions only in presenting a more coarsely fibrous 

 radiation, which is not rigid and uniform, but somewhat fan-shaped. These fibres 

 are both positive and negative, the two being in close contact. Of two spreading, 

 fan-shaped bundles of fibre, one shows the blue of the positive crystal (on inserting 

 the quartz teinte sensible) and the other the yellow or yellowish red. This is 

 characteristic of thomsonite. In the same slide are smaller amounts of other 

 zeolites, apparently okenite with positive elongation and nx'solHc. 



Two sections. 

 " Af/e. Cabotian; probably the Beaver Bay diabase. N. H. w. 



