430 THE GEOLOGY OP MINNESOTA. 



[Diabase. Thomsonite and prehnite. 



e in small amounts in one section. 



Titan tic has resulted from alteration of ilmenite, but it is very rare in the section 

 examined. 



MYr\.s/7<> forms spherulitic clusters liable to be mistaken for chlorastrolite, from 

 which it is distinguishable by its pleochroism (light green and white). 



There is also a finely fibrous, translucent, colorless mineral, whose appearance 

 and structure in general is like that described in No. 140, which may also be 

 thalite. 



Three sections. 



Age. Cabotian(?) N. n. w. 



No. 572. DIABASE. ( Gabbroidal. ) 



The rock of Scovill's point, Isle Royale. 

 .Re/. Annual Report, x, page 53. 



Meg. Diabase considerably decayed, amygdaloidal with a flesh-red zeolite 

 resembling laumontite, medium-grained brownish. 



Mic. The feldspars are thickly charged with kaolinic substances, and they have 

 sometimes lost their triclinic characters. 



Olivine is altered as usual, and consists now of the substance which is usually 

 seen as the product of such alteration, surrounded by brown borders and crossed by 

 brown cleavage cracks or fissures in which the ferruginous oxide has gathered. 



Delessite, a green, finely radiated mineral is gathered in other places, remote 

 from the olivines, which between crossed nicols shows a parallel extinction and a 

 bluish color. 



Augite is in fine grains that antedated the feldspar. There is a much ferru- 

 ginated mineral which shows an ophitic relation to the feldspar, which if not a later 

 generation of augite is probably a glassy remnant of the magma, now entirely 

 devitrified. The original augites are well preserved. 



One section. 



Age. Cabotian(?) N. H. w. 



No. 573. THOMSONITE AND PREHNITE. 



From the trap at Scovill's point, Isle Royale; picked up on the beach. 

 Ref, Annual Report, x, page 53. 



Meg. Mostly radiated, nearly white, pebbles, averaging nearly an inch in 

 diameter, occasionally tinted with gray (when fresh). 



Mic. The fibration consists apparently of two (or three) minerals, one much 

 coarser than the other. The coarse fibres are visible in natural light, but the fine 

 ones are not. Between crossed nicols their double refraction when viewed perpen- 

 dicular to their elongation is low the section being less than .03 millimeter in 



