. The Crystal Falls iron-bearing district of Michigan. Iron ores; Geology; Geology, Stratigraphic; Geology, Stratigraphic. 112 THE CRYSTAL FALLS IKON BEARING DISTRICT. TIIK ELLIPSOIDAL STRL'CTUUE IN THE iMETAISASALTS. Upon examining the flat surfaces of many of the lavas one is immedi- ately struck by their resemblance to a conglomerate formed of round bowlders, all of the same kind of rock, lying in a matrix of very small quantit}' and of very different color. Fig. 7 is a sketch showing a portion of such a lava flow. I find that these ellipsoidally parted rocks have been called "massive
. The Crystal Falls iron-bearing district of Michigan. Iron ores; Geology; Geology, Stratigraphic; Geology, Stratigraphic. 112 THE CRYSTAL FALLS IKON BEARING DISTRICT. TIIK ELLIPSOIDAL STRL'CTUUE IN THE iMETAISASALTS. Upon examining the flat surfaces of many of the lavas one is immedi- ately struck by their resemblance to a conglomerate formed of round bowlders, all of the same kind of rock, lying in a matrix of very small quantit}' and of very different color. Fig. 7 is a sketch showing a portion of such a lava flow. I find that these ellipsoidally parted rocks have been called "massive conglomerates," and the blocks have been spoken of as "bombs" in the manuscript notes of some of the men who have worked among them. The latter term was undoubtedly due to the resemblance of the ellipsoids to the spindle-shaped pieces of lava m. ^ - H â ^fmm'^:rA Fig. 7.âSkitch of ihe siirliice dl' Uic «Hitci'ui» of au basalt, aliuwiufi the gtin^ral rliaiaiter of tlit- ellipsoids find matrix- which one finds around the modern volcanoes. Ellipsoidal basalt is very common throughout the Hemlock volcanic area. It is found most frequently in isolated ledges. However, it is also associated with and grades into non- ellipsoidal varieties. In one good exposure it is overlain by a fragmental scoriaceous mass which separates it from another mass of similar ellipsoidal basalt. While the scoriaceous portion may represent the brecciated surface of a lava flow, it is not so considered, biit is presumed to be a tuff" deposited upon the flow represented by the ellipsoidal basalt. According to this view the ellipsoidal basalt is on the surface. In another exposure an ellipsoidal basalt overlies a bed of water-deposited clastic I'ock. There is no passage between the two kinds of rock. The contact between the two is an undu- lating one, and is marked by a mass of schistose material about 2 inches thick and similar to that which is between the ellipsoids.
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