30 THE CAMBRIAN AND ORDOVICIAN DEPOSITS OF MARYLAND 



along these lines just as in the north and south structural troughs they 

 thin along the edges of the trough. 



The greatest sea withdrawal marks the systemic boundaries, but the 

 advance and retreat of the seas within the systems would naturally not be 

 uniform throughout the basins of deposition. Therefore one area will 

 have deposits which are represented in another area by a stratigraphic 

 hiatus. As a result of this, the complete section cannot be found at any 

 one place but it is a composite one made up of units from many places. 

 The disregard of this fact has delayed the recognition of many new 

 formations heretofore, as in the case of the so-called Ozarkian. 



Under the conception of little elevation and often slight erosion of the 

 Eopaleozoic land masses, the stratigraphic unconformities are not strongly 

 marked even though the time interval may have been great. The bedding 

 planes of strata belonging to distinct formations are usually parallel and 

 the detection of such unconformities is most difficult. The f aunal method 

 of discrimination is a sure one, provided the occurrence and range of the 

 faunas are well known. Another method is in the comparison of detached 

 sections and noting the gradual interpolation of other strata between two 

 formations with persistent lithologic characters. In the discrimination 

 of the Ordovician rocks of Maryland, this latter method is extremely 

 useful, as several of the formations developed in states to the north and 

 south are represented here only by their overlapping margins. 



In the classification of Paleozoic rocks, as commonly recognized, the 

 Cambrian and Ordovician systems forming the subject of this volume 

 comprise the Eopaleozoic. In his revision of the Paleozoic systems, Ulrich 

 has proposed and defined two new systems, the Ozarkian and Canadian, 

 which occupy the interval between a slightly restricted Cambrian and a 

 more modified Ordovician system. Brief descriptions of these new 

 systems were read at the Baltimore meeting of the Geological Society 

 of America in 1909, but were not published until 1911. In 1910, in his 

 Paleogeography of North America, Schuchert adopted both of the new 

 systems, crediting them to Ulrich, and introduced a third for the Cin- 

 cinnatian rocks hitherto classified at the top of the Ordovician. The 



