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Show A lurlotis lllii, rul. ft-KMllLE SA1IK.I0M' RKVIAUKABLR H)U TIIK MRTAU8 IN IT. One of tlie best marked clnrnctcrls. tics ol most rocks is their rigidity, but a few spei les are known which aie more llexlble thrul wood anil readily bcuil under slight pressure without breaking The itacolumltc, llexlble sandstone, is most abundant of these rocks und Is found principally in Brazil, although a similar fornntioii is met with in miiner oih cither I icMitl-i In smtll quantlilcs. 1 he llexlbibty of llacoliimitc is readily understood when the stone is exiiulurd with the microscope All sindstone consists of separate gr ilns of s ind trans formed into solid roelc b) some cement-iiiLr cement-iiiLr nnteriii which has percolated In solution between the gnlns This cement varies in lis nature, nnd may consist of carbomlo of lime, oxide of iron, silica oruun other 'ubstuuees, even Including In a lew rare instances native silver and copper. All these ordinary sandstones ate rigid aud brittle, but in itacolumite the grains nrc ec mented together by a mineral cloaely resembling mica or sericite These minerals, being ipilte flexible in themselves, them-selves, conur tlie Mine properly upon the sandstone as a whole. the way in which the cementing material was Introduced into tie itaco lumite Is not eas) of explanation Mka and sericite are not soluble and could not have been deposited by water, like calcile or silica It Is must probable-that probable-that they vvcro originally introduced In the form of clay or some similar material, und afterward metamorphosed by heal, pressure and super heiled steam into miciceous mineral. Instances In-stances of n similar change of one mineral species Into another aie very common |
