560 THE GEOLOGY OP MINNESOTA. 



[Diabase. 



No. 821. DIABASE. 



Taylor's Falls. 



Ref. Annual Report, x, page 116. 



.]///. Similar to Nos. 820 and 822, but the ophitic augites have changed to a 

 yellowish-green material. There are also splashes of red color on the specimen. 



Mic. Similar to the more altered parts of Nos. 820 and 822. The augite has 

 all disappeared, and the feldspar also is highly altered. One poor section. 



Age. Cabotian. u. s. G. 



No. 822. DIABASE. 



Same rock as No. 820, but taken near the surface in order to show the brownish-gray weathered spots 

 which appear on the exposed surface of No. 820. 

 Ref. Annual Report, x, page 116. 



Meg. The rock, No. 820, is flecked with a spotted faint rustiness, the colored 

 spots appearing near the natural surface of the ground, and at the surface. The 

 spots are from a sixteenth to a quarter of an inch in diameter, approximately circular, 

 and appear to be due to the dissemination of iron oxide from some oxidizing mineral 

 having much iron, since at the centres of the spots may be seen frequently a fine 

 powder resembling limonite. At other times the spots are simply a light gray color 

 pervading the grains of the darker gray mass. 



Mic. There seems to be no special difference between this rock and No. 820, 

 except that hematite can be seen more abundant in some parts of the slide, and 

 nil cite is noticeable in large areas. 



It is probable that the discoloration, in spots, is due to the action of artificial 

 heat on the surface, for these rocks were formerly covered with a forest which has 

 largely been destroyed by fires. Further, when these spots are near the thin weather- 

 coating, they blend with it, all the minerals acquiring a rustiness. The green spots 

 mentioned in connection with No. 821 are not due to the same cause, but more 

 likely are caused by a deep-seated alteration of some mineral, perhaps pyroxene. 



Age. Cabotian. N. H. w. 



No. 823. DIABASE. 



"Pebbles from the conglomerate forming the lower part of the bluff excavated by the railroad where it 

 crosses the highway south of Taylor's Falls." 

 Ref. Annual Report, x, page 117. 



The pebbles are of the general diabasic rocks of the region; they are con- 

 siderably decayed and do not show the lustre-mottling common to Nos. 820 to 822, 

 and they are also finer grained than the most of the adjoining rocks. 



Mic. The section shows a rock composed of minute irregular laths of plagiwl<i*<' 

 and a mass of alteration products, chief among which are iron ores, chlorite and an 

 opaque, gray substance. One section. 



Age. Pebbles in the Upper Cambrian. u. s. G. 



