140 W. J. McGee — Three Formations of 



whole sometimes so similar to disintegrated granite or gneiss as 

 to be distinguished only with difficulty. It reaches its best 

 development toward the base of the formation, and especially 

 in the smaller valleys and ravines or on the subjacent gneissic 

 surface ; and it is not found in • perfection along the lines of 

 the larger rivers. Its petrography and distribution alike justify 

 the inference that it is granitic debris, not far transported. 



4. By far the larger part of the quartzose sand, whether in 

 homogeneous beds or intermingled with other constituents of 

 the formation, consists of rounded quartz grains of doubtful 

 origin, but evidently worn by transportation ; a smaller part 

 consists of angular quartz grains and flakes such as might be 

 produced by impact of masses during transportation ; a yet 

 smaller part is made up of rudely crystalline grains such as re- 

 sult from the disintegration of vein quartz ; and the least im- 

 portant element in volume, though it is locally conspicuous and 

 significant, consists of more or less perfect crystals of quartz 

 such as might form the residue of disintegrated granite after 

 the solution and removal of the less obdurate constituents. All 

 of these sources are doubtless represented in the sands of the 

 Potomac. 



5. The clay occurs in minute flakes (sometimes of crystalline 

 form) in the arkose, in rounded and irregular pellets and balls 

 up to an inch or more in diameter in the arkose and sand beds, 

 in lenticular or sometimes continuous bands intercalated with 

 sand, and again in considerable beds exhibiting more or less 

 definite structure planes; but whenever pure it is clean, plastic, 

 kaolin-like, and evidently derived from a common source, and 

 the smaller flakes retain the shapes of feldspar crystals undis- 

 tinguishable from those of the adjacent Piedmont gneisses. 

 The clay in the larger masses it is true appears to be thoroughly 

 triturated, and was evidently deposited in finely divided con- 

 dition ; and the pellets and balls appear to have been washed 

 from such beds and redeposited in conjunction with other 

 materials ; but the structural differences between the pseudo- 

 crystalline and the water-laid phases of the clay are no greater 

 than would inevitably result from the trituration and assort- 

 ment accompanying the breaking up of gneiss or arkose and 

 the separation of the materials of unlike specific gravity and 

 solubility. 



6. The heterogeneous and ever varying accumulations of 

 composite character which constitute the larger part of the for- 

 mation are, qualitatively, just such as would be formed by the 

 assortment and deposition of the different materials by power- 

 ful currents ; but the quantity of coarse materials in the Poto- 

 mac formation is greater than would result from simple admix- 

 ture of the disintegrated gneiss of the Piedmont zone and such 



