330 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1933 



clays, and gravels of this formation, the early Cretaceous Potomac 

 group, must conclude that these deposits were laid down on the ancient 

 continent in river or swamp beds rather than in the sea. For here 

 are found bones of dinosaurs, the giant land reptiles of the past; 

 trunks of trees changed to flint and jet, and many leaf impressions 

 between the clay layers. 



The higher northwestern part of Washington is built upon the 

 Piedmont plateau, which is the planed-off surface of former moun- 

 tains whose rocks represent the oldest period of earth history. These 

 rocks, well represented in Eock Creek and westward toward the 

 Blue Eidge, have passed through so many changes of structure due 

 to pressure and fracturing that their original aspect can only be 

 surmised. They consist mainly of crystalline rocks, so-called because 

 they were produced by the slow cooling and crystallization of an- 

 cient lavas or by the remelting and subsequent cooling of sedimen- 

 tary strata, such as mudstones. Granites, the most common of the 

 granular crystalline rocks, and schists, made up of thin layers of 

 changed, compressed sedimentary strata, form here the predominat- 

 ing outcrops. Besides the characteristic granite-gneiss of the eastern 

 Piedmont, which is well exposed throughout Eock Creek Park, other 

 interesting rocks outcrop just west of the park, among them diorite, 

 which weathers into green soapstone, and which has been used as a 

 building stone for various houses in Washington. 



To study other rocks of the Piedmont, the geologist visits the 

 Potomac gorge at Great Falls, where a banded granite, called the 

 Carolina gneiss, outcrops extensively. The Potomac itself follows 

 the so-called master joint planes of the great blocks into which the 

 granite is divided, and within a short distance its course changes 

 direction several times at almost right angles. Even these very 

 hard rocks, because of their exposure to the weather for such count- 

 less ages, usually appear at the surface as crumbling masses of quartz 

 grains, mica flakes, and clay, but along the walls of the stream gorge 

 where weathering has not been at work so long, their real nature is 

 perceptible. In the gorge of the Potomac another erosional feature 

 is conspicuous, for here the polished rock ledges contain rows of 

 pot holes, or circular excavations, sometimes as much as 7 feet wide 

 and 10 feet deep, worn in the hard bedrock by the water whirling 

 sand and boulders around in crevices until the cavities are produced. 

 Dikes and quartz veins containing just enough gold to be annoying, 

 intrusions of igneous rock from deep in the crust, faulting and the 

 results of long weathering and erosion are other geologic phenomena 

 plainly visible in the gorge. 



Perhaps the most interesting field of geologic study close to Wash- 

 ington is the region of folded mountains, known as the Newer Ap- 



