. The Crystal Falls iron-bearing district of Michigan. Iron ores; Geology; Geology, Stratigraphic; Geology, Stratigraphic. 120 THli CRYSTAL FALLS IROX-KEARING which ill cross section give elHptical iigures, l>ut whose iudetenninate down- ward extension sliows them to be cohimns. The rounding of the cokimns, which were presumably originally prismatic, he ascribes to dynamic action- He also sug'srests that elliijsoidal masses could result from a similar dynamic modification of a mass of lava parted into shorter prisms, or even ellipsoids. In the description of the eruption at >&
. The Crystal Falls iron-bearing district of Michigan. Iron ores; Geology; Geology, Stratigraphic; Geology, Stratigraphic. 120 THli CRYSTAL FALLS IROX-KEARING which ill cross section give elHptical iigures, l>ut whose iudetenninate down- ward extension sliows them to be cohimns. The rounding of the cokimns, which were presumably originally prismatic, he ascribes to dynamic action- He also sug'srests that elliijsoidal masses could result from a similar dynamic modification of a mass of lava parted into shorter prisms, or even ellipsoids. In the description of the eruption at >>antorin, Fouque' mentions a viscous lava exuded in the form of a mass of blocks. These blocks, tum- bling o^'er one another as the mass is pushed from l^ehind, have accumulated in a rough pile, PI. XII. Fouque climbed these piles of block lava shortly after their production, and noticed the breaking off of pieces from the sides, due to the cooling and contraction of the individual blocks.^ In general this character agrees well v.'ith that of the aa lava of Hawaii, as descriljed by tlie late Prof. J. D. Dana.' He describes the formation of the blocks as due to the slow for- ward movement and contemporaneous breaking up of the viscous lava. The surface contrasts with the ropy surface of the more liquid pahoehoe. The aa is as a rule compact as compared with the })alioehoe, though the exterior "is roughly cavernous, horribly jagged, with projections often a foot or more long that are bristled all over with points and ; From the illustrations of this lava (see fig. 10, taken fi-om Dana) the blocks may be seen to be, while irregular, still in general distinctly rounded. This is the shape which viscous material would naturally tend to take when sitbjected to the rolling action attendant upon the onward motion of the stream of which they form an outer portion, or in certain cases the entire thickness. This is clearly shown from the following quotation from Dana's de
Size: 2355px × 1061px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, bookpublishe, booksubjectgeology
