PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 251 



Porphyryte.] 



/ 



they represent the thalite phase of alteration in these rocks, to which, evidently, 

 olivine has contributed largely. The close alliance between this thalite stage and 

 the bowlingite stage is evident from the fact that here they occur in the same mass, 

 under similar physical conditions, but in rock which consolidated at different rates 

 by reason of differences of cooling. 



The amygdules in this rock are feebly translucent in the thin section, even in a 

 very thin section. The section contains many. It is hardly possible that this feeble 

 translucency can be attributed to the accidental orientation of all the fibres, of all 

 the amygdules in the same position, and that position such that they uniformly 

 present optic axes, and for that reason transmit little or no light. They are 

 remarkably in contrast with the thomsonite of No. 163A, but they seem to represent 

 the finely fibrous pink mineral associated with thomsonite in that rock. This fine 

 mineral is the only one that is known to occur at Terrace point, varying to the char- 

 acters assigned by Peckham and Hall to lintonite* This mineral is mesolite. (See 

 Nos. 535A, B and C.) 



Age. Manitou. N. H. w. 



No. 197. PORPHYRYTE. (Diabase.} 



"A reddish brown rock, closely jointed, and also breaking sharply with a conchoidal fracture; very rough 

 exteriorly, i. e., with sharp projecting angles that tear the boots, but not porous or open; forms the point and 

 coast line first east of Good Harbor bay, east of No. 195." 



Ref. Annual Report, ix, page 52. 



Meg. A very fine-grained, compact, brownish rock, carrying a few scattered, 

 gray plagioclase phenocrysts, which are less than one-eighth of an inch in length, 

 and usually not more than half that length. There are also a few irregular areas of 

 chlorite in the rock. 



Mir. The small porphyritic plagiodases occur in a groundmass, the most evident 

 crystals of which are plagioclase microliths. The phenocrysts are considerably 

 altered, and, while no characteristics determining absolutely their species were found, 

 there are indications that they are labradorite. The groundmass aside from the pla- 

 gioclase microliths, is composed of feldspathic material, chlorite, magnetite, //cuititite, 

 and a brownish material which perhaps represents unhi<liri<lnliz<'<l HHUJIIK/. There 

 are also minute, brightly polarizing grains in the groundmass which are probably 

 augite. A few olivines are present in the slide. 



One section. 



Age. Cabotian. u. s. G. 



Remark. For reasons that are elsewhere given it is supposed that the strati- 

 graphic horizon eastward from Good Harbor bay, at the lake shore, is that of the 



* American Journal of Science, third series, vol. xix, p. 122, February, 1880. Republished in the Eighth Annual Report of 

 the Minnesota Survey, p. 166. 



