Bulletin - Geological Survey, Issue 34

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Page 17 - ... periods when marine life abounded and the sediments were entirely calcareous. Black slates owe their blackness to carbonaceous matter, probably derived from the decomposition of marine organisms on the sea floor. Red slates owe their color to the access of ferruginous matter from the land, and purplish slates to an admixture of such matter and a green magnosian mineral (chlorite) of secondary origin.
Page 17 - The alternation of beds of slate with beds of quartzite or grit ("ribbons," "hards," altered sandstone) corresponds to the alternation of extremely fine clayey sediments, derived from the waste of granitic land masses, with sandy sediments consisting of coarser material from the same source. The repeated alternation of such fine and coarse sediments is attributed to the alternation of calm water, favorable to the deposition of fine material, with strong currents that bring coarse sediments...
Page 19 - ... combines some of the features of a crystal with some of those of a tissue. Extremely thin sections of slate transverse to the cleavage sometimes show this interlacing of the two sets of scales on their attenuated edges. It is to this microscopic texture that slate largely owes its peculiar properties. This crystalline fabric may inclose in its meshes any sedimentary particles of quartz, zircon, feldspar, kaolin, or other minerals which were not or could not be made over into mica or secondary...
Page 40 - Knox dolomite is shown by paleontological evidence to include both the upper part of the Cambrian and the lower part of the Ordovician.
Page 22 - The strongest slate stands highest in weathering qualities, so that a flexural test affords an excellent index of all its properties, particularly if the ultimate deflection and the manner of rupture be noted. The strongest and best slate has the highest percentage of silicates of iron and alumina, but is not necessarily the lowest in carbonates of lime and magnesia. Chemical analyses give only imperfect conclusions regarding the weathering qualities of slates and do not satisfactorily explain their...
Page 19 - As mica crystallizes in columnar crystals, and as the plates or scales due to its molecular structure are transverse to the crystal column, and as a slab of slate consists largely of parallel scales of mica it may be said to correspond when held horizontally to such a crystal held vertically.
Page 18 - There was also, under the combined presence of moisture and the effect of pressure and heat, both the heat which must have been generated by the pressure and that which pervaded the strata at the depth at which they were buried, such a chemical recombination of the silica, alumina, potash, iron, and water of the feldspar, kaolin, and iron of the shale as to generate new...

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