Publisher's Synopsis
Excerpt from The Geological Map of Illinois
Purpose of a geological map - The position of the natural eco nomic products which exist within the crust Of the earth in any region, is determined either by reason of the physical conditions pres ent at the time Of formation Of the rocks in which they are contained, or by reason Of subsequent dynamic changes. Thus beds Of coal, limestone, sandstone, clay, etc., which may be termed primary eco nomic products, are where they are in the earth's crust by reason of certain peculiar local conditions which have been present in that locality at some past time in the history Of the earth; the present position of most metallic mineral deposits, on the other hand, and also Of petroleum, is usually determined first, by certain structural features within the earth's crust. Such as faults and folds, which have in?uenced the secondary segregation Of these deposits; and second, by the peculiar character and textures Of the rocks which are affected by the folding and faulting.
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