. [Reports]. ence in the number of orogenic movements towhich the series have been subjected; (3) discordance of beddingof upper series and foliation of lower; (4) relations with eruptives;(5) difference in degree of crystalization; (6) basal conglomer-ates; and (7) general field relations. Where none of these phenomena can be recognized, uncon-formities have been inferred upon paleontological grounds, althothere has always been some doubt as to the reliability of thiscriterion. Paleontologists often recognize, in thin beds of con-glomerate, uncomformities which can only be inferred from thech
. [Reports]. ence in the number of orogenic movements towhich the series have been subjected; (3) discordance of beddingof upper series and foliation of lower; (4) relations with eruptives;(5) difference in degree of crystalization; (6) basal conglomer-ates; and (7) general field relations. Where none of these phenomena can be recognized, uncon-formities have been inferred upon paleontological grounds, althothere has always been some doubt as to the reliability of thiscriterion. Paleontologists often recognize, in thin beds of con-glomerate, uncomformities which can only be inferred from thechanges in fauna from one formation to another. Such an uncon-formity may represent a hiatus in the faunal development, duethe incursion of more highly developed forms, and still have nosignificance as regards the continuity of sedimentation. ?Principles of North American Pre-Cambrian Geology, 16th An. Report U. S. G. S., Pt. 1pp. 724, 725. Missouri Bureau of Geology and Mines. Vol. IX., Series 2, Plate Residual flint boulders. The remnants of some formation which, with these excep-tions, has been completely removed from this area. STRUCTURES. 85 The_ recognition of unconformities in the area under discus-sion is based altogether upon the phenomena as outlined by VanHise in the report above referred to. Laurentian-Lamotte Unconformity.—Reference was made inthe preceding chapter to the unconformity existing between thepre-Cambrian and younger formations. In this area there is noevidence that the Huronian ever existed between the Laurentianand the Lamotte, altho in the vicinity of Pilot Knob and Iron Moun-tain there is some evidence of its former existence. The fossilscontained in the upper part of the Lamotte sandstone and the baseof the Bonneterre dolomite seem to indicate that the Lower Cam-brian is also wanting. If these conclusions are true, there was a very long period of erosion between the time that theLaurentian igneous rocks were formed and the subse
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