REPORT OF THE SECRETARY. 73 



ous spur extends into the little valley of the branch that forms the principal affluent 

 of Rock Creek, and that another spur (from which the loam has been washed, lay- 

 ing bare the coarse materials of the inferior member) extends southeastward beyond 

 the terrace-scarp toward the upper angle in the course of Rock Creek. The red loam 

 of the upper member was derived mainly from the Piedmont gneiss of the upper 

 reaches of Rock Creek ; while the lower member consists of sand and some loam from 

 the same source, well-rounded pebbles and cobble-stones from the Potomac formation, 

 angular or slightly water-worn fragments of quartz from the veins of that material 

 cutting the gneiss both within the Park and beyond its limits, bowlders of gneiss, etc. 



" The Potomac formation is a series of sands, clays, anu gravels extending from tho« 

 Roanoke to the Delaware, but best developed along the Potomac River, in honor of 

 which the formation was christened. The age, determined through paleo-botauy by 

 Professor Fontaine, is early Cretaceous; determined from vertebrate paleontology by 

 Professor Marsh, is Jurassic; and as determined by physical geology the formation 

 represents the beginning of the Cretaceous. Along its westernmost margin the 

 formation is usually represented, by outlying patches of gravel commonly crowning 

 eminences; and this is the character displayed in the Park. Five small areas only 

 occur in the reservation : There is a remnant retaining the original structure crown- 

 ing tlie second greatest eminence in the northwestern part ; there are two small rem- 

 nants, one certainly displaying the original structure upon the eminence occupied by 

 the Holt mansion in the southeastern corner of the reservation; there is a fourth 

 remnant, which may be in place, but is probably a residuum let down and disturbed by 

 the decay of the subjacent gneiss, mid-length of the sonth western boundary; and 

 there is another small area, which is certainly residual in the northeastern portion. 

 These remnants and others of like character beyond the limits of the Park are of 

 especial interest in that their cobble-stones were exte'jsively used by aboriginal men 

 for the manufacture of rude implements. Modern man also utilizes the cobble-stones 

 extensively for road-making and other jiurposes. 



"The Piedmont gneiss is a vast complex of crystalline rocks extending from Ala- 

 bama to New Jersey. Many rock varieties are recognized within the complex; but 

 they have not yet been systematically differentiated throughout any considerable 

 part of the terrane. Within the Park the prevailing rocks are schists varying in 

 composition from place to place, and varying also in dip and strike. In general the 

 dip is high, sometimes nearly vertical, and the prevailing strike is northerly and 

 southerly. The gneiss is the prevailing formation of the Park. It is overlain in part 

 by alluvium and by the Columbia formation, as well as by the isolated remnants of 

 the Potomac formation; and elsewhere it has been decomjiosed to a considerable 

 depth so that it is concealed by a mantle of materials derived from its own destruction 

 either in place or carried down slopes by gravity and the wash of storm waters. This 

 mantle of decomposed rock may be 20, 50, or even more feet in thickness, and proba- 

 bly averages no less than 15 or 20 feet over the entire reservation. So profound has 

 been this decomposition of the crystalline rocks that exposures occur only in the 

 steeper bluHs where Rock Creek has corroded rapidly during the later Neocene, Pleis- 

 tocene, and recent times. The rocks of the Piedmont belt are seldom sufficiently 

 firm, tough, and durable to yield valuable building stones, and within the Park tkey 

 give little promise in this direction. At three points only is the promise even fair: 

 In the extreme northwestern corner of the reservation, toward the northern end of 

 the old quarry mid-length of the eastern side, and in the old quarry opposite Adams's 

 Mill. 



" Within the Park, as beyond its limits, the Piedmont gneisses are frequently inter- 

 sected by veins of quartz. These range from sheets but a fraction of an inch thick 

 to great nuisses many yards across. Some of the more conspicuous examples have 

 been mapped. No law governing the trend or inclination of these veins is indicated 

 by these exposures, and no such law has thus far been formulated; but although the 

 relation of the quartz veins to the gneisses is not apparent, there is an obvious rela- 



