Identifier: birdsnature9101unse (find matches)
Title: Birds and nature
Year: 1900 (1900s)
Authors:
Subjects: Birds Natural history
Publisher: Chicago, Ill. : A.W. Mumford, Publisher
Contributing Library: University of Illinois Urbana-Champaign
Digitizing Sponsor: University of Illinois Urbana-Champaign

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acture, i. e. itexhibits little rounded concavities andconvexities resembling a shell in shape. The mineral is quite brittle. Someemeralds even have the annoying habit ofbreaking of their own accord soon afterremoval from/ the mine. This can beprevented by warming them graduallybefore exposing them to the heat of thesun or other sudden heat. Beryl and its varieties, like tourmaline,are dichroic, i. e. the stones exhibit differ-ent colors when viewed in different direc-tions. This dichroism can sometimes beobserved by the naked eye, but often notwithout the aid of the instrument knownas the dichroscope. When seen it fur-nishes a positive means of distinguishinga true stone from any glass imitations. The varieties of Beryl have none ofthe brilliancy of the diamond and there-fore depend wholly on their body colorsand their lustre for their beauty and at-tractiveness. Fortunately they exhibitthese qualities as well by artificial light asby daylight. Ordinary Beryl is a mineral of com- 1::
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LIBRARY ^VERSIFY (tf ILUNOIS paratively common occurrence, beingoften found in granitic and metamorphicrocks. That of common occurrence is usuallytoo clouded and fractured to be of usefor gem cutting. There are many local-ities, however, where Beryls of gemquality occur. The finest emeralds in the world comefrom Muso, a locality in the UnitedStates of Colombia, seventy-five miles N.N. W. of Bogota. It is a wild and in-accessible region and the mining of thegems is a precarious occupation. Theemeralds occur according to Bauer in adark, bituminous limestone which isshown by fossils to be of Cretaceous age.As emeralds in other localities occur onlyin eruptive or metamorphic rocks, itseems probable that the Muso emeraldshave washed in from an older formation.The emerald bearing beds are horizontal,overlying red sandstone and clay slate.Calcite, quartz, pyrite and the rare min-eral parisite are other minerals found as-sociated with the emerald. The mannerof working these emerald mines is t

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