PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 413 



Basalt.] 



No. 544. BASALT. ( Zirkelyte, globuliferous. ) 



Grand Portage island. In a belt crossing the island east and west, but probably only a phase of rock No. 

 545. It is the last to run under the water at the west end, and at the east end it is the topmost rock in the 

 bluff. Its thickness may be twenty feet. 



Ref. Annual Report, x, page 15. 



Mty. A curious, dark globuliferous trap-rock, disintegrating readily, the hard 

 globules, which are of stony structure (not minerals), and nearly black, rolling out 

 like shot, or bullets, and covering the ground after the rest of the rock is rotted to a 

 greenish soil. The rock itself is chloritic and dark green. 



Mir. The rock was consolidated before it crystallized. As a consequence there 

 is a large amount of undifferentiated magma, which, however, is devitrified. It now 

 embraces handsome spangles of magnetite, which is the only determinate mineral 

 connected with the magma remnants. Other spaces of exceedingly irregular outline 

 are filled with a pale yellowish, fibrous alteration mineral that resembles thalite. 

 The most of the rock itself consists of small augite crystals or fragments of crystals, 

 generally granular and sub-rounded, but not entirely so. Amongst these are a few 

 fine plagioclases, and occasionally in the centre of a lot of alteration products is a 

 little quartz. These, with considerable calcite, make up the rock. 



Five sections. 



Age. Manitou. N. H. w. 



No. 545. BASALT. 



"A bed of fibrous green trap, passing through the centre of the island [Grand Portage] and forming its 

 highest parts, and also producing a long sloping beach on the southeast side running under the chalcedonic 

 amygdaloid [No. 543]. In the centre of the island it appears like a burnt scoria or slag, due, perhaps, to the effect 

 of Nos. 543 and 542, though these have been removed in the central part of the island, appearing now only along 

 the south and southwest shores." 



Ref. Annual Report, x, pages 45, 46. 



Meg. A dark-gray, very fine-grained, heavy, compact diabasic rock. 



Mtc. The section is composed largely of small, thickly crowded augite prisms. 

 With these are numerous small Magnetite, crystals. These two minerals are set in a 

 sparse background of feldspar. This is in comparatively small areas of later date 

 than the other minerals and enclosing them poikilitically. The different feldspar 

 ai'eas are not sharply separated from each other; they infrequently show twinning, 

 and their species was not determined. Scattered through the section are many green 

 to yellow areas composed of a mineral in minute scales (serpentine). Some of these 

 areas possibly represent old olivine phenocrysts, but many of them seem to be rather 

 an altered part of the rock, possibly oi'iginally glassy. 



Two sections. 



Age. Manitou. u. s. o. 



