PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 433 



Thomsonite. Prehnite and copper.] 



No. 575. THOMSONITE. 



v 



From the beach on the north side of Isle Royale, about two miles southwest from Locke's point. 



Meg. Pebbles of zeolitic minerals weathered from the trap-rocks of the region. 

 They resemble those of No. 573, except that the majority of them have a tendency 

 toward red, becoming pinkish and purplish, and in this respect they approach more 

 nearly to the pebbles that have been distributed widely from Grand Marais under 

 the name thomsonite. Still, they fall far short of being so brilliantly colored. Their 

 structure is divergently fibrous. 



Ifie. The microscopic characters are the same as already noted for thomsonite 

 under Nos. 161B, 535 and 573. So far as examined, these pebbles are without prehnite, 

 but the slide shows some delessite. Two sections. 



Age. Cabotian(?) 



Remark. In this large collection some of the pebbles are white, or cream- 

 white, and some are variously blotched with a green substance similar to lintonite 

 The white pebbles are probably thomsonite, perhaps sometimes mingled with 

 prehnite. A thin section made from one of the sub-translucent green pebbles, 

 without evident fibrous structure, proved to consist wholly of the same mineral, 

 but massively and finely fibrous. 



A few other pebbles in the same collection are evidently of prehnite. 



Analysis of No. 575. This zeolite was analyzed by Prof. J. A. Dodge, with the following result. The 

 material used seems to have been considerably altered by weathering: 



Silica, 45.47 per cent. 45.47 



Alumina, 21.01 



Oxide of Iron, 3.60 



Lime, % 24.09 



Magnesia, .16 



Potash, .21 



Soda, .93 



Water, .83 



Carbonic Acid, - 4.10 



21.01 



3.60 



18.87 



.16 



.21 



.93 



.83 



Garb, of Lime, 9.32 



100.40 per cent. 100.40 



Regarding the carbonic acid as united with a portion of the lime, we have carbonate of lime 9.32 per cent, 

 and we have left of the lime 18.87 per cent, which is to be considered as in combination with the silica, alumina, 

 etc., as it stands in the right-hand column. 



N. H. W. 



No. 576. PREHNITE AND COPPER. 



Prom a little bay at the eastern extremity of Pish island, north side of Isle Royale from the beach. 

 Ref. Annual Report, x, page 53. 



Meg. The prehnite has served as a cement to angular fragments of amygda- 

 loidal diabase, probably derived from a vein in the near vicinity, not seen. The 

 color is light, grayish green, and the fracture is angular and vitreous. 



Mic. The single section at hand serves only to indicate the high double 

 refraction, which is sufficient to bring out colors of the fourth order, owing to its 

 thickness. One section. 



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



2tf 



