526 THE GEOLOGY OF MINNESOTA. 



[Diabase. Conglomerate. 



Mic. The section shows a very fine aggregate of quartz, chlorite, Muscovite and 

 dust-like material. Considerable calcite is present, and this is usually in larger 

 grains than the other minerals. A few rut He needles are present. 



One section examined. 



Age. Keewatin. IT. s. G. 



No. 749. DIABASE. 



From a dike cutting No. 748. This dike runs north and south and is thirty to forty feet in width. 

 Ref. Annual Report, x, page 92; Bulletin ii, page 117. 



Mey. A medium-grained, dark-greenish rock evidently containing much horn- 

 blende. 



Mi<\ M. E. Wadsworth's description of this rock is as follows:* 



" The section shows the usual structure of altered diabases, and is composed of 

 divergent, quite largely altered feldspars (plagioclase), with the interstitial altered 

 tiiii/ife, itKiynetite and secondary viridite, chlorite. It/o/ite, hornblende, etc. The mag- 

 netite is associated with leucoxeneand yields on measurement the same angles as that 

 seen in No. 758." 



One section examined. 



Aye. Probably Keweenawan. u. s. G. 



NO. 750. CONGLOMERATK. 



North side of Dyke lake; N. E. % S. E. % sec. 28, T. Ki-6 W. 

 Ref. Annual Report, x, pages 92, 95; Bulletin ii, page 122. 



Meg. A gritty, greenish rock, resembling No. 744. Two rounded pebbles of 

 considerable size, one of " greenstone " and the other of white flint, or jaspilyte, are 

 present. 



Mic. M. E. Wadsworth's description of this rock, which he included under the 

 term porodyte, is as follows:f 



"The sections are composed of fragments of diabase, melaphyr and t/imrtz, with 

 interstitial material composed originally of their debris. One of the melaphyr frag- 

 ments, evidently, was nearly in a glassy state in its original condition. The non- 

 quartzose material retains its structure largely, but beyond this it has been altered 

 to chlorite and colorless micaceous scales. The feldspars retain their outlines, but 

 are entirely filled with the colorless mica scales, and show aggregate polarization. 

 1'i/rite, in striated eubes, is very abundant as a secondary constituent of the rock. 

 The quartz probably came from some granitoid rock." 



Two sections examined. 



Age. Ogishke conglomerate at the base of the Upper Keewatin. IT. s. G. 



