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Jzoic, or those destitute of any discovered relics of life ; and 

 Paleozoic, or those entombing the remains of the earth's most 

 ancient extinct forms of once living beings. 



The Atlantic slope of Pennsylvania includes all these three 

 systems of strata. Where the azoic strata display their maxi- 

 mum amount of crystalline structure or metamorphism, they often 

 simulate the true ancient hypozoic or gneissic rocks so closely, 

 and they are indeed so identical with them in mineral aspect and 

 structure, that the observer is baffled in his attempts to distin- 

 guish the two groups lithologically ; nevertheless, it clearly ap- 

 pears, as the sections illustrating this country prove, that they are 

 distinct systems, occupying separate zones, susceptible of inde- 

 pendent definition on the geological map. 



At the time of the first construction of the general geological 

 map of the State, the true limits separating the hypozoic or 

 gneissic from the azoic or semi-metamorphic rocks were but 

 vaguely understood, and the State geologist did not venture to 

 define them on the map, but shaded the one system into the 

 other, indicating, however, what he has since proved, that the 

 true gneissic rocks, in their southwestward course, pass out of 

 the State at the Susquehanna, only a short distance north of its 

 southern boundary, while the azoic, or talco-micaceous group, as 

 a genuine, downward extension of the primal, paleozoic series, 

 widens progressively going westward, until, from a very narrow 

 outcrop at the River Schuylkill, it occupies at the Susquehanna the 

 whole broad zone south of the limestone valleys of the Conestoga 

 and Codorus streams in Lancaster and York counties. Since the 

 revival of the field work of the survey, the dividing limit of these 

 two sets of metaraorphic strata has been traced and mapped with 

 precision. To the southwest of the Susquehanna it has never, 

 it is believed, been pursued through Maryland and the other 

 southern States, though one may readily discern it in going north- 

 Avard or westward from Baltimore, or ascending the Atlantic 

 slope in Virginia. In Maryland it crosses the Baltimore and 

 Susquehanna Railroad about twelve miles north of Baltimore, and 

 it is intersected by the Baltimore and Ohio raih-oad a little east of 

 Sykesville ; it crosses the Potomac above Georgetown, and the 

 James River in Virginia, west of Richmond. The line of bound- 

 ary is, however, not a simple one, but is intricately looped, in 



