32 THE CAMBRIAN AND ORDOVICIAN DEPOSITS OF MARYLAND 



pods types which, up to the present, are entirely absent in Cambrian 

 faunas. The Canadian faunas introduce a wealth of graptolites, true 

 orthoids as distinguished from Billingsellidae brachiopods, the first 

 ostracods, and the first of the coiled cephalopods. The Ordovician fauna 

 is at once distinguished from the Canadian by the first appearance of 

 pelecypods, the first of the unquestionable bryozoans. and the first true 

 crinoids. 



The following table is introduced to illustrate graphically the various 

 usages of the Silurian and related terms concerned in this volume : 



Although the list of papers dealing with the geology of the parts of 

 Maryland concerned in this volume is quite lengthy, the number of 

 students whose observations have advanced the knowledge of the strati- 

 graphy and paleontology of the region is comparatively small. 



In 1885, Mr. H. R. Geiger began the study of the Paleozoic rocks 

 along the Potomac River in western Maryland and West Virginia and 

 in 1886 and 1887 extended his work eastward down the Potomac River 

 and for some distance southward over the Great Valley region of Virginia. 

 In 1888 he began work on the Harper's Ferry quadrangle and after 

 several months study came to certain conclusions regarding the relations 

 of the sandstones and associated formations in the Blue Ridge and South 

 Mountain to the limestones of the Great Valley which are not held to-day. 



In 1890 Mr. Arthur Keith undertook a reexamination of the Harper's 

 Ferry quadrangle and as a preliminary result of his studies read a paper 

 in joint authorship with Mr. H. R. Geiger, before the Geological Society 

 of America on " The Structure of the Blue Ridge near Harper's Ferry." 1 

 The next year he published a short notice on " The Geologic Structure of 

 the Blue Ridge in Maryland and Virginia," * and his final results appeared 

 in the Harper's Ferry folio No. 10, Geologic Atlas of the United States. 



In 1892 Mr. Charles D. Walcott made an examination of the Blue 

 Ridge and South Mountain region and definitely determined the Cam- 

 brian age of its quartzites. A statement of the results of this investigation 



1 Bull. Geol. Soc. America, vol. ii, 1891, pp. 155-164, pis. iv, v. 

 3 American Geologist, vol. x, 1892, pp. 362-368. 



