PREFACE 



The present volume is the seventh of a series of reports dealing with 

 the systematic geology and paleontology of Maryland, the Devonian, 

 Lower Cretaceous, Upper Cretaceous, Eocene, Miocene and Plio-Pleisto- 

 cene deposits having already been fully described. 



This volume is devoted to a consideration of .the Cambrian and Ordo- 

 vician deposits and their contained faunas. The calcareous strata making 

 up a considerable part of these two systems are so intimately united in the 

 Appalachians that they have long remained unseparated under the name 

 Cambro-Ordovician limestone, or, as in Maryland and Virginia, the 

 Shenandoah limestone. It was therefore thought eminently fitting to 

 combine their consideration in one volume and to disregard in the title 

 the Ozarkian, the Canadian, and other possible systems which have been 

 suggested in recent years. 



Upon the completion of studies on the Cambrian and Ordovician rocks 

 of Virginia in 1909, published as Bulletin 2 A of the Virginia Geological 

 Survey under the title " Cement Resources of Virginia west of the Blue 

 Ridge," the writer, upon the invitation of the late Dr. William Bullock 

 Clark, undertook the preparation of the Maryland volume dealing with 

 the systematic geology and paleontology of the Cambrian and Ordovician 

 systems. As this work included the mapping of these strata in areas both 

 west and east of the Blue Ridge, and as only a few weeks were available 

 each summer for the necessary field work, the volume has been long 

 delayed. However, this delay has proved fortunate, for during the 

 interval of its preparation the knowledge of Cambrian and Ordovician 

 stratigraphy has progressed so much that it is hoped fewer mistakes 

 will now be recorded. 



During the preparation of the Mercersburg-Chambersburg folio descrip- 

 tive of the region in Pennsylvania, just north of the Maryland state line, 



