GEOLOGIC EXHIBITS IN NATIONAL ZOOLOGICAL PARK — BASSLER 277 



form the rock mantle as distinguished from the hard underlying 

 rocks. Here this is also an acid soil because composed mainly of the 

 acid mmeral silica derived from the acid rock granite beneath. 

 Veins of hard insoluble quartz cross both types of rock but remain 

 so unchanged upon weathering that they can easily be detached from 

 the enclosing weathered rocks. Associated with such quartz veins in 

 the granite here and elsewhere around Washington are interesting 

 minerals, particularly hornblende, and in a few places small particles 

 of gold. The feldspar portion of the granite, however, often 

 weathers into pure kaolin. Now and then quartz crystals {rock 

 crystals) line cavities in veins where they have had enough space to 

 form. Although not visible at this particular outcrop, dikes, frac- 

 tures filled with crystalline material from intruded igneous rocks, are 

 visible elsewhere in the park. At the near end of this outcrop may 

 be seen a depression in the schists filled with boulders, and the sand 

 beds of the Lower Cretaceous Potomac formation. These probably 

 fill an ancient stream valley, but at any rate the outcrop shows the 

 same unconformity between the pre-Cambrian and Mesozoic noted 

 on Harvard Street (pi. 8). Concretions, rounded boulderlike objects 

 formed of concentric layers of clay or other material segregated 

 around a central nucleus, are occasionally found in these sandy layers. 



Transverse movements within the earth's crust are also recorded in 

 this section, as the schists on the south side of the intrusion show a 

 distinct break along a fault line parallel to the street. Tliis can be 

 demonstrated visually by the dislocation suffered by a quartz vein 

 exposed in the bank (pi. 8). Harvard Street is now at hand, and 

 this excursion ends close to the starting place, the unconformity on 

 the terraced slope. 



In conclusion, it will be noted that in this short field trip the follow- 

 ing items of physical geology are illustrated : Weathering, the rmi-off 

 or surface water, ground water, the work of the wind, igneous rocks, 

 sedimentary rocks, diastrophism, metamorphism, mountain structure 

 and land forms, comprising, with the exception of glaciation, work 

 of the sea, earthquakes, and vulcanism, the major subjects of this 

 science. For historical geology, rocks and deposits of the pre-Cam- 

 brian eras as recorded in the several granites, gneisses, and schists, 

 various formations of the Paleozoic studied by means of pebbles from 

 the Appalachians, boulder deposits of the Mesozoic Potomac forma- 

 tion, the sands and gravels of the Cenozoic and, lastly, the modern 

 flood plain alluvial sediments, and the Indian quarries, comprise the 

 available record. 



So many geologic items are visible in this small part of the Na- 

 tional Zoological Park, that the following alphabetized index of 

 those italicized in this article seems an appropriate conclusion. 



