144 THE GEOLOGY OP MINNESOTA. 



[Lava. Diabase. 



No. 50. LAVA. (Vesicular.) 



Duluth. Next east of No. 41. 



Ref. Annual Report, ix, pages 17, 18. 



Meg. A reddish-brown, aphanitic rock with many vesicles which are in part 

 empty and in part filled with calcite. The specimen also contains fragments of 

 other rocks. A few small, red, porphyritic feldspars are also seen; dip, E. 10 S. 18; 

 extent about 150 feet. 



Mic. The section is very thick, but on its edges it can be seen to be made up of 

 a transparent background in which are crowded specks and grains of a red material, 

 probably hematite. Where the section is thicker this red material renders it com- 

 pletely opaque. Under a high power and in polarized light the background breaks 

 up into a very finely crystallized aggregate of quartz. It is possible that some of 

 this background is feldspathic, but no feldspar could be determined. In small areas 

 the red material becomes much decreased, or entirely lacking, and here the quartz 

 is in larger interlocking grains. The few porphyritic feldspars seen in the slide are 

 much altered, but seem to be orthoclase. While the original nature of this rock is 

 not clear, it still seems likely that it was a highly vesicular, probably rather acid 

 lava. 



One section. 



Age. Cabotian. u. s. G. 



No. 51. DIABASE. ( Porphyritic. ) 



Duluth. From a point, the next one beyond No. 50. Extends 200 feet. Sometimes rises twelve feet per- 

 pendicular from the water. 



Ref. Annual Report, ix, page 18. 



Meg. A fine-grained, brown rock made up of small, reddish feldspars, a dark, 

 almost black, substance, and epidote. There are a few porphyritic red feldspars, 

 and some larger areas with narrow, red periphery and dark interior. These latter, 

 perhaps, represent large tabular feldspars which have been largely replaced by 

 chlorite. One of these is three-fourths of an inch in diameter. 



Mic. The section is composed of small lath-shaped plagioclases, which are much 

 altered and reddened. Between these crystals are quartz, magnetite, calcite, chlorite 

 and some epidote. Apatite needles also occur in the rock. The most characteristic 

 feature of the section is the large amount of quartz in distinct grains. This fills in 

 the spaces between the feldspars, and occasionally a considerable area is seen to be 

 covered by quartz of like orientation; thus the quartz sometimes embraces the feld- 

 spars poikilitically. One section has on its edge a part of one of the porphyritic 

 feldspars. This is reddened, much altered and contains some chlorite, and on the 

 edge of the section it passes into an area of almost pure chlorite. It thus seems very 

 probable that the large black areas already mentioned in the hand specimen represent 



