Dave, my geology is mixed up to a “fare-thee-well”. But thanks to you I know now about my local stone friends:
“…Potosi Agate, Missouri Lace Agate, Missouri Agate… It was formed in place in the Potosi Formation of Late Cambrian Age from acid groundwater perculating through the rock, which also produced the red, sticky clay residue called “terra rosa” that this drusy quartz is found in.” So not pelagic at’all. The terra rosa is all that’s left of the acid-melted dolostone matrix the agates have formed in. The agate comes first, when and how I don’t know, then the druse. I happen to be polishing one up right now. It’s a beaut.
I apologize for my OCD. Thanks! Anybody coming to read these late comments is surely going to be dissappointed.

http://au.answers.yahoo.com/question/index.php?qid=20070219132245AAaFG6L