Gabbroid Rocks of Minnesota. — Wincliell. 215 
Adams and others have used the term anorthosyte in a very 
wide sense, making it nearly synonymous with gabbro, noryte, 
or even diabase, in different cases. Thus Adams says:* 
"Beide (i. e., coarse and fine anorthosyte along the Shipsaw 
river, Canada), zeigen sehr deutlich eine ophitische oder Dia- 
base Structur," etc. 
In this article the word plagioclasyte is used in a more re- 
stricted sense, so as to be no longer an equivalent of gabbro, 
but merely a variety of the latter just as Iherzolyte is a variety 
of peridotyte. Thus plagioclasyte is a member of the family of 
gabbro rocks and is related to t3^pical gabbro in just the same 
way as pyroxenyte.f That is by a gradual decrease of one of 
the essential constituents (feldspar) gabbro finally becomes 
pyroxenyte, while by a similar gradual decrease of the other 
essential constituent (pyroxene) gabbro passes into plagiocla- 
syte. Plagioclasyte thus stands at one end of the series of gab- 
bro modifications and pyroxenyte at the other. A mixture of 
plagioclasyte and p\-roxen}-te in about equal proportions 
would produce a typical gabbro. The analogue of plagiocla- 
s}'te in the potassic feldspar series has been called sanidinyte. 
Occurence. In Minnesota, typical plagioclasyte occurs at 
several points along the southern edge of the great gabbro 
area, as well as further north at Bellissima lake, Little Sagana- 
ga lake, and elsewhere. Along the coast of lake Superior (i. e. 
along the southern edge of the gabbro mass), plagioclasyte has 
been reported* from ten localities, the most important being; 
near Encampment island,§ Split-rock point, Beaver bay, Bap- 
tism river and at Carlton peak. It is from this last locality, 
where typical plagioclasyte is said to form the whole upper 
part of a hill three hundred yards high, that the material for 
this study has been obtained. Typical plagioclasyte is known 
*Ueber das Norian oder Ober-Laurentian von Canada. F. D. 
Adams: Neues Jahrb. f. Mineral, etc. Bd. VIII, p. 451. Cf. also: 
Report on the geology of a portion of the Laurentian area: Geol. 
Surv. Canada Ann. Rep. 1896. Vol. VIII. 
tA name first proposed by T. S. Hunt for the pyroxenic masses 
occurring in the great gabbro areas of Canada, but now generally used 
in the sense proposed by Geo, H. Williams to designate a granitoid 
rock composed essentially of pyroxene, with no olivine, nor feldspar. 
See Amer. Geol.. July, 1890, p. 47. 
J A. C. Lawson: Anorthosytes of ISIinnesota Coast of Lake Supe- 
rior: Geol. Nat. Hist. Surv. Minn. Bull. No. 8, 1893. 
§See the map. 
