992 THE GEOLOGY OP MINNESOTA. 



f Anomalous megascopic facts . 



No. 1528. Very similar to No. 1530. This is about at the horizon of No. 1527, but the specimen shows no 

 basaltic jointage. Largely from volcanic glass sand, but showing original variations in the sand, some of it 

 being still isotropic. Some of the quartz is coarse and may have been originally fraginental, but it is now 

 wholly interlocked with the matrix; one specimen. 



No. 1527. The globular structure is visible on most of the freshly fractured surfaces. This is basaltically 

 jointed. 



The whole thickness of these beds is, visibly, but about five feet. Between them 

 and the first outcrop of the Pokegama quartzyte is a short interval unexposed, the 

 rock of which is unknown. The quartzyte (Nos. 1525, 1525A and 1526) is supposed 

 to underlie the foregoing, but it may overlie. In this quartzyte are isotropic globules 

 and areas which are ascribable to volcanic glass, also tourmaline which indicates 

 solfataric action. Compare the microscopical descriptions. 



4. The description of the Pokegama quartzyte published in the Eighteenth 

 Annual Report (pages 15-18) combined with a re-examination of specimens collected 

 (Nos. 1525-1535) leads to the belief that it is not wholly below the iron ore, but 

 cotemporary with and later than some parts of the ore. It was also later (on the 

 evidence of pebbles which it contains) than some rock (not from the Archean) which 

 appears to be from the Keweenawan. In short, the conglomeratic parts of the 

 Pokegama quartzyte (No. 1532), which parts, so far as known, are near the upper 

 limit of the quartzyte, are perhaps of the age of the Puckwunge conglomerate, and 

 therefore later than an important eruptive part of the Keweenawan. The ore 

 at the Diamond mine, eastward from Prairie River falls, is in a quartz sandstone 

 (No. 1534) and is overlain by a fine red unctuous shale (No. 1533) that is like some 

 seen in the Keweenawan.* This ore has not the distinct taconitic structure seen in 

 the hematite at Prairie River falls (No. 1527) although it fades out in the fragments! 

 sand in the form of roundish grains as if it had a taconitic (globular) origin. While 

 it lies higher than the quartzyte, that which is seen at Prairie River falls and has a 

 basaltic structure may belong below the quartzyte. 



5. There is a suggestive slatiness parallel with the general dip, which is like 

 that known to pervade surface igneous rocks. It is comparable with that seen in 

 the igneous felsitic slates of Pennsylvania, discussed by Prof. George H. Williams,f 

 and by Dr. F. Bascom.:}: This is also fully described by Mr. Spurr, in Bulletin x, 

 pages 167-172. The minuteness of his description is so exact that one can easily 

 attribute some of the characters to spherulitic parting planes in a surface lava, and 

 in general the whole cleavage to that of a streamed surface lava on slow cooling. 



6. A volcanic epoch on the south side of lake Superior has been described on 

 the Penokee iron range (the parallel of the Mesabi) by Prof. C. R. Van Hise (No. 1939), 



* See figure 2, Eighteenth Annual Report. 



^American Journal of Science, third aeries, vols. 44 and 46, 1892 and 1898. 

 lSv.ilf.lin cxxxvi, United States (jcul. Survey, 1890. 



$ Bulletin of the Geological Society of America, vol. iv, p. 435, 1893 (distributed in 1894) ; Afonoymplt xix, ( ". iS. Gcol. Survey, 

 1892, pp. 380, 379. 



