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If there are volcanoes around now or in the past
OK JB - tell us one place where there were no volcanoes in the past (he's failing a bit, so we should probably cut him some slack.)
this is a glacial erratic
As near as I can tell from the geologic map of the US there are mostly sedimentary rocks in Cedar County. (The Iowa Geologic Survey is too cheap to put a geologic map on its site.) BUT, the occurrence of galena, a crystalline mineral, is evidence of intense mineralization in the area of eastern Iowa. So there is magma under them there plains.
Hydrothermal or other vados water activity produced these. They are crystals deposited on and in the host rocks - shale or black limestone in the last three picture.
EVERYONE knows black shale or limestone means they are very high in organic content! D'Oh!
Quote from: JimBob on 12/03/2010 18:37:46EVERYONE knows black shale or limestone means they are very high in organic content! D'Oh!Yup! Even I knew that.