124 THE CAMBRIAN AND ORDOVICIAN DEPOSITS OF MARYLAND 



glomerates occur in place of the cauliflower chert which is so conspicu- 

 ously developed elsewhere in the valley. Occasionally these basal edgewise 

 conglomerates become silicified, leaving very interesting chert blocks in 

 the soil exhibiting the conglomeratic fragments as pure chalcedonic silica, 

 or in some cases showing them leached out of the rock entirely. 



Middle Stones River. The middle part of this limestone is so readily 

 distinguished from the upper and lower divisions by its fossils and lith- 

 ology that it might well be ranked as a distinct formation. The Middle 

 Stones Eiver differs conspicuously from the other divisions in the pres- 

 ence of massive beds of dark, subgranular limestone interbedded with the 

 more typical dove-colored, fine-grained, pure limestone. Fine-grained, 

 pure, fossiliferous limestone weathering so as to show numerous bands 

 of black chert form a characteristic part of this division. In the absence 

 of outcrops an easily recognized criterion for the determination of the 

 middle division is the presence of small blocks of chert in the soil. This 

 chert is usually black, at least the inside of the blocks when broken show 

 up as black. Furthermore, the Middle Stones Eiver is generally more 

 fossiliferous than the other divisions and the typical fossil, the large 

 coiled gastropod shell Maclurites magnus, can usually be found in it after 

 a little search. 



The best outcrops for fossils of the Middle Stones River are in the 

 quarries around Chambersburg, Pennsylvania, where the fauna listed 

 below was collected and determined by Ulrich ( Chambersburg-Mercers- 

 burg Folio). All of these species, however, have been noted in the 

 corresponding band of outcrop in Maryland, although all have not 

 occurred at a single place. . 



Fauna of the Middle Stones Eiver Limestone in Southern Pennsylvania 

 and in Maryland 



Tetradium syringoporoides (Ulrich) 

 Hebertella borealis (Billings) 

 Hebertella vulgaris (Raymond) 

 Dinorthis platys (Billings) 

 Bucania sulcatina (Emmons) 

 Maclurites magnus (Lesueur) 

 Lophospira bicincta (Hall) 

 Ampyx halli (Billings) 

 Leperditia fabulites (Conrad) var. 



