144 THE CAMBRIAN AND ORDOVICIAN DEPOSITS OF MARYLAND 



in the underlying Christiania zone and second because the guide fossils 

 of still lower members of the Chambersburg limestone, Nidulites and 

 Echinospherites; here reappear in well developed, apparently typical 

 specimens. The number of species found in this bed is rather large but 

 only two apparently characteristic forms, the cephalopods Orthoceras 

 arcuoliratum and Cyrtoceras camurum, are described in the present 

 volume. 



SECTIONS OF THE CHAMBEESBUKQ LIMESTONE. Extended outcrops of 

 the Chambersburg limestone are so few in Maryland that in order to get 

 an idea of the formation it is necessary to study the exposures in con- 

 tiguous states. In Maryland there are only two places, namely, at Pines- 

 burg Station and Wilson, where a detailed section of the formation can 

 be studied, but in Pennsylvania there are numerous localities where all 

 the members of the formation can be seen to great advantage. 



As noted above the formation varies considerably from east to west 

 and it has been found convenient to study the divisions in sections 

 exposed east of the Martinsburg shale belt and west of the same area. 

 Throughout the Maryland basin the datum plane for the base of the 

 Chambersburg is the Upper Stones Eiver dove limestone which is every- 

 where developed and easily recognized. The various sections quoted below 

 were prepared by E. 0. Ulrich, with whom the writer was associated in 

 the study of this limestone. Most of them have since been published in 

 the Chambersburg-Mercersburg folio. 



Sections East of Martinsburg Shale Belt in Pennsylvania. Although 

 the Chambersburg limestone is named from Chambersburg, Pennsyl- 

 vania, its outcrops at this place are too few or incomplete to give a good 

 idea of the entire section. The Christiania and Greencastle beds forming 

 the upper part of the Chambersburg are well exposed here, the Echino- 

 spherites bed is well shown along the railroad, but the Nidulites bed is 

 mostly covered and the Tetradium cellulosum bed is not seen at all. This 

 section, which is given below, is particularly interesting in showing the 

 development of the Greencastle bed and also in exhibiting the different 

 faunal zones of the lower part of the Martinsburg shale. 



