132 THE CAMBRIAN AND ORDOVICIAN DEPOSITS or MARYLAND 



diagram on page 131, which shows that' the diminished thickness in 

 Maryland is due mainly to the absence of the lower divisions. . 



AREAL DISTRIBUTION. The area of outcrop of the Chambersburg lime- 

 stone in Maryland is less than that of any other formation west of the 

 Blue Eidge. Faulting along the east side of the Martinsburg shale belt 

 has cut out the Chambersburg limestone everywhere in this part of the 

 valley except in the very northern part of the state, and a very small patch 

 in the great bend of the Potomac River at the southern edge. 



West of the shale belt the Chambersburg outcrops in an almost con- 

 tinuous band extending from Pinesburg Station northward to the Penn- 

 sylvania state line. West of this there is a second band of outcrop which, 

 however, is cut out by faulting before half the state is traversed. 



The outcrops east of the Martinsburg shale belt give very little idea 

 of the formation, and in fact its presence can be recognized only by a few 

 traces of cobbly, dark-blue limestone showing here and there along the 

 roads and in the fields. These exposures are so incomplete that it is 

 impossible to work out any adequate idea of the sequence and character of 

 the beds. The few exposures, however, indicate the presence of the 

 Echinospherites and Nidulites beds described below. 



The two lines of outcrop in the western half of the valley afford a better 

 opportunity for the study of the formation. At Pinesburg Station a good 

 exposure of the rocks may be seen, particularly the Echinospherites bed. 

 The road running north from Pinesburg Station to Pinesburg passes 

 along the strike of these rocks and numerous outcrops may be seen on 

 either side of it. At Wilson the Echinospherites bed is not so well exposed, 

 but the overlying Nidulites bed is well shown in the bluff overlooking 

 Conococheague Creek. North of this is a rather broad area of the forma- 

 tion, but here the rocks are hidden to a great extent by recent stream 

 gravels. The line of outcrop west .of the Pinesburg- Wilson area extends 

 only half way across the state on account of faulting, and exposures are 

 extremely few and poor throughout its length. 



FAUNAL ZONES. The researches of Ulrich in the Appalachian Valley 

 of southern Pennsylvania, in which the present writer had an opportunity 



