416 THE GEOLOGY OF MINNESOTA. 



[Diabase. 



Age. Puckwunge (basal Potsdam). 



Hi'inark. The greatly altered condition of the silicates in this conglomerate can 

 be attributed to hot waters that resulted from the. immediately succeeding surface 

 lavas which were cotemporary with the formation in which they occur, and which 

 are entirely distinct structurally from the intrusives. N. H. w. 



No. 550. DIABASE. 



Grand Portage island. From a dike, thirty-four feet wide, cutting the quartzyte and conglomerate, near 

 No. 541. 



Kef. Annual Report, x, page 46. 



Meg. A medium-grained, dark-gray diabasic rock. It contains some plagioclase 

 crystals considerably larger than the most of the feldspars, thus giving a porphyritic 

 aspect to the rock. 



Mic. The section shows a much changed rock filled with confused dirty altera- 

 tion products which are usually brownish. Playioc/tiNc and mit/ifc are pretty well 

 preserved, and it seem possible that olivine, as well as unindividualized matter, was 

 originally present. 



One section. 



Age. Manitou(?) u. s. G. 



No. 551. DIABASE (m- basalt). 



From a dike crossing the southeast corner of Grand Portage island. 

 Ref. Annual Report, x, pages 46, 94, 112. 



Meg. Medium-grained, dark-gray, heavy diabase. 



Mic. The section shows lath-shaped feldspars, any if c, iiiriyitrfifr and a confused 

 mass of dirty, greenish alteration products. The feldspar shows equal extinction 

 angles on either side of the albite twinning line as high as 25, indicating labradori/c. 

 The augite is in grains rather than in plates and some of it is earlier than, or of about 

 the same date as, the feldspar, while some is later than the feldspar. Magnetite, 

 especially in rough rod-like forms, is abundant. There are also some areas which 

 seem to represent altered olivines. It seems probable that some of the areas of 

 alteration products carne from an unindividualized base. 



One section. 



Age. Manitou(?) 



Remarks. This rock is said to constitute a dike which crosses the southeast 

 corner of the island. It is thus later than and cuts the igneous beds of this part of 

 the island. The conglomerate and quartzyte do not occur on this part of the island. 



Leaving out of consideration the actual age of the rocks of Grand Portage island 

 and their relations to the rocks of the adjacent main land, it may be said that this 

 island appears to hold three different rock masses, which may be arranged p-ov /*///- 



