52 THE CAMBRIAN AND ORDOVICIAN DEPOSITS OF MARYLAND 



East of the Blue Eidge province, sandstone metamorphosed into quartz- 

 ites, limestone changed into marbles, and shales into slates or schists, 

 outcrop in small patches at numerous points on the Piedmont Plateau. 

 Although fossil evidence regarding the age of these latter strata is in 

 most cases wanting, it is believed that they represent at least portions of 

 the three phases of deposition farther west. Their correlation, however, 

 cannot be confirmed until the geologic history of the Piedmont province 

 has been studied in detail. The present volume is therefore devoted more 

 to the discussion of the stratigraphy and paleontology of the Cambrian 

 and Ordovician rocks of the Blue Ridge and more western provinces, 

 although for the sake of completeness, brief notes on the Piedmont strata 

 of apparently the same age are introduced in their appropriate places. 



CAMBRIAN SILICEOUS FORMATIONS 



Although the siliceous Lower Cambrian rocks outcrop in long but 

 interrupted stretches from Vermont to Alabama, they differ so greatly 

 in character and sequence from place to place that none of the formations, 

 if any were ever so extended, are unquestionably recognizable throughout 

 the whole Appalachian province. A few widely separated areas have been 

 studied in detail, but, on account of difficulties in correlation, differing 

 sets of local names had to be applied to the formations distinguished in 

 each. The excellent exposure in the gorge of the Potomac Eiver where it 

 breaks through the Blue Ridge early attracted the attention of geologists 

 to the Maryland-Virginia section. The sequence of formations here 

 determined and named has been traced to the north across the state into 

 Pennsylvania and proved satisfactory, and is generally accepted as the 

 standard for the Lower Cambrian in the north middle Appalachian 

 region. These formations and their thicknesses arranged in geologic 

 order, are as follows : 



Table of Maryland Lower Cambrian Siliceous Formations 



Feet 



Antietam sandstone. Coarse grained white to bluish sandstone 800 



Harpers shale. Bluish gray sandy slates and schist 1200 



Weverton sandstone. Massive white and purple sandstone and quartzite. . 800 

 Loudon formation. Dark slates, sandstone, shales and marbles 500 



