MINERALOGY AND PETROLOGY. 991 



Anomalous megascopic facts.] 



Anomalous megascopic facts. 1. The iron-bearing rock is, as a whole, non-elastic 

 in its present condition. It is only by inference that it can be said ever to have 

 been clastic. This applies more forcibly to the rock known as non-productive 

 taconyte than to the rich ore lenses, for there is in the ore bodies an overspreading, 

 general, bedded structure which indicates sedimentation, especially in the granular 

 portions. Both above and below the iron-bearing member are plainly clastic rocks, 

 and occasionally some clastic grains are seen near the upper and lower surfaces of 

 the iron-bearing rocks. These facts indicate the general prevalence of detrital 

 agencies. It leaves room only for the inference that the iron-bearing rock itself was 

 of a peculiar composition. 



2. The unproductive iron-bearing rocks are divisible into two kinds, viz.: (1) 

 A massive homogeneous, but rudely sheeted or bedded, gray or brownish-gray, 

 fine-grained rock (Nos. 1688, 1692), in which appears very little or none of the char- 

 acteristic globular or any other fragmental structure; and (2) The rock which is 

 distinctly granular with colored or limpid round globules, one of the characteristic 

 structures of taconyte (No. 852B). There are intermediate structures, apparently, 

 and the globular forms, or others resembling them, occasionally are seen in No. 1. 

 The globular form constitutes, when the iron is fully developed, the bulk of the "soft 

 ores" of the western part of the Mesabi range. No. 1 prevails at the eastern end of 

 the Mesabi range, where it also varies to a jaspilyte. 



3. There is a marked basaltic, columnar structure which has been fully described 

 by Mr. Spurr in Bulletin x, pp. 164-167 (Nos. 64S, 65S). His description in all respects, 

 except as to his assignment of cause, could be applied to numerous basaltic obsidians 

 or lavas. He says: "The typical jointing of the iron-bearing rocks has all of these 

 characteristics," i. e., the characteristics of the basaltic jointing of "igneous rocks of 

 an effusive or intrusive nature." An instance of this columnar structure was noted 

 at Prairie River falls in 1889 by the writer, and three of the specimens collected 

 there by him (No. 1527) are shown, from photograph, on plate IV, figure 1. The 

 columnar structure involves both globular-granular and non-globular forms of the 

 tacouyte.* The order of parts from Nos. 1530 to 1525, in descending order, when 

 interpreted in the light of the latest developments, is as follows: 



No. 1530. A coarse, evidently fragmental, rusty, siliceous, somewhat vesicular rock. The five samples of 

 this rock collected, and now in the museum of the University, consist of globular taconyte, but with some 

 spreading, irregular masses, suggesting breccia or lapilli in the midst of volcanic sand. One specimen shows a 

 banding that might be due to variation in sediments. See plate III; also, page 739. Originally a volcanic sand. 



No. 1529. Peculiar "streamed" and brecciatod mixture of chalcedonic quartz, globular taconyte and 

 geodic quartz, the last tilling original elongated cavities, which are embraced entirely in a casing of white 

 "chalcedonic" quartz. This seems to be from the superficial part of a lava flow where volcanic (glass) sand 

 accumulated amidst the breccia. Some parts of this rock are like the banded jaspilyte of the Vermilion range; 

 four specimens. 



*Other interesting features of this rock at Prairie River falls are mentioned in the annual report for that year (the 

 Eighteenth) p. 15. 



