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| | Limestone Encyclopedia Article, Definition, History, Biography (Site not responding. Last check: 2007-10-20) |
 | | Limestones often contain variable amounts of silica in the form of chert or flint, as well as amounts of clay, silt and sand as disseminations, nodules, or layers within the rock unit. |
 | | Limestone is especially popular in architecture, and many landmarks around the world, especially in North America and Europe, are made primarily of the material. |
 | | Limestone is used as a facade on some skyscrapers, but only in thin plates for covering rather than solid blocks. |
| www.alienartifacts.com /encyclopedia/Limestone (641 words) |
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