GEOLOGIC EXHIBITS IN NATIONAL, ZOOLOGICAL PARK BASSLER 269 



such rounding, is shown in the light blue blocks (2) ; a sandstone 

 formed on an ancient sea beach is the yellow material (5) illustrat- 

 ing stratification and closely arranged 7'ipple niiarhs. The intrusive 

 igneous rocks are represented by the blocks of coarse-grained granites 

 composed of the essential minerals^ quartz and feldspar^ with small 

 fragments of accessory minerah, white muscovite mica and black 

 biotite mica (3). Metamorphic rocks originating from both ig- 

 neous and sedimentary rocks are present in the column, one (1) being 

 micaceous schist with occasional pebbles, the latter indicating their 

 sedimentary origin, and another (4) thin-banded gneiss derived 

 from the metamorphism of granite. 



ROCK CREEK AND VICINITY 



As we pass through the Harvard Street gates. Rock Creek comes 

 into view, with many evidences of the phenomena of running water 

 and atmospheric erosion, as well as the deposition of sediments. Most 

 of the exhibits in the park are of natural origin, but by blasting 

 away the rocks and in other ways some of them have been rendered 

 more conspicuous. Sprmgs bubble forth here and there, arising 

 either in the porous sedimentary beds at the top of the hills or in 

 the joint planes of the igneous rocks, which thus serve as water bearers 

 {aquifers). Here and there in protected areas in the stream, gelat- 

 inous masses of diatoms may be found, their remains in the muddy 

 bottom recalling the diatomaceous earth found abundantly on the 

 coastal plain around Washington. The subjects of sedimentation 

 and sedimentary rocks may be studied on the plain to the left at the 

 base of the cliff recently containing the bear pits (fig. 1, P). Here, 

 Rock Creek in its annual flood periods, as seen in the outcropping 

 edges along the stream banks, has deposited in horizontal layers 

 alternating strata of sand, silt^ and clay,, the last sometimes com- 

 pressed into shale. All these various forms of stream sediments are 

 called alluvium., and the resulting plain, an alluvial plain. The 

 sandy layers when examined under a lens are mainly composed of 

 angular fragments of quartz, indicative of their deposition in water 

 which, buoying them up, did not permit much wear. An occasional 

 layer does contain rounded grains showing origin by wind distribu- 

 tion, in this case forming an eolian sandstone. The stream-borne 

 material of this plain mixed with the leafy mold of present-day 

 trees is sufficiently fertile to be classified as a transported soil. This 

 small section of stratified rocks also shows several thin beds of vege- 

 tation composed of leaves and twigs representing the annual deposit 

 of the nearby trees. These layers now slowly being compressed into 

 peat., and this, with the pressure of future years, into coal,, are sep- 

 arated by sand strata marking the intervening coarse deposits of 



