21G TRANSACTIOJsS OF THE [aPKIL 5^ 



to be one vast elongated uplift. The greater part of this is 

 made of crystalline rocks, most probably of Lauren tian Age, 

 though the presence of areas underlaid by younger Archrean 

 strata may not be improbable. The western boundary of this 

 valley is found in the Adirondacks of northern New York, also 

 of Laurentian rocks, but farther southwest this margin is made 

 by folds of Paleozoic strata, and of these the Oneida conglom- 

 erate, or Shawangnnk grit, and Medina sandstone are the most 

 prominent, principally for the reason that, from their greater 

 hardness, they have better withstood the erosion which has cut 

 away the less resistant Lower Silurian limestones and slates 

 which form the floor of tiie great valley throughout nearly its 

 entire extent. To certain features of these softer rocks I would 

 first call your attention. The Hudson River shite, which ordi- 

 narily lies next the Oneida or Medina, though also occurring in 

 detached areas, surrounded by the limestones, is either a true 

 clay slate or a very sandy slate varying to a sandstone, and 

 sometimes designated as gray wacke; locally it becomes appreci- 

 ably calcareous. "When a true slate, which is its general charac- 

 ter, it contains but few fossils, as will be seen in the fact that, 

 up to the present time, not a single organism has been detected 

 in it throughout its entire area of several hundred square miles 

 in New Jersey. The limestones which it geologically overlies are, 

 for the most part, magnesian, though some areas of pure lime- 

 stone are recorded. This doiomitic rock is also very barren of 

 fossils; it may, however, generally be recognized by its litholog- 

 ical and chemical features, which seem to be quite persistent. It 

 has been referred to the Calciferous Epoch of the New York Ge- 

 ologists, and it would be ditiicult to prove that this is not its 

 horizon. Locally, other fossiliferous limestones occur in the 

 Great Valley, but, taken as a whole, must be regarded as entii'cly 

 subordinate in amount to that of the magnesian. Underlying 

 this limestone, and always found in greater or less development 

 between it and the crystalline rocks to the east, is a sandstone, or, 

 more generally, quartzite, of immense thickness along the south- 

 western extension of the valley, less abundant to the northeast, 

 which answers in geological position to the Potsdam of northern 

 New York, and has generally been regarded as of that age. 



About Luray, Virginia, located in Page Valley, as the Appala- 

 chian Valley is there locally known, the magnesian limestones 

 are seen in many outcrops along the Hawksbill Creek. There 

 appears to be a synclinal fold in the rocks just west of the creek, 

 for along its western bank the dip is 37° W., while GOO feet away 

 the dip is 40° 8. E., and about half a mile up the stream they 

 are bedded nearly horizontally. East of these outcrops there is 

 an extensive tract of country covered with loose pebbles and 



