268 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 39 



formed, as today, in low flats or swamps of that time. Sometimes 

 they are thick enough to have been of economic importance in the 

 early history of our country. Occasional clay balls, sometimes sev- 

 eral feet in diameter, occurring in the sand layers above, indicate 

 probably the course of ancient waterways and today are a fertile 

 source of fossil leaf impressions. Fragments and sometimes entire 

 trunks of petiif^ed trees may also be found in sandy layers. These 

 trees are true petrifactions^ as the original wood structure has been 

 replaced by mineral matter which in this case is silica. More fre- 

 quently the tree trunks have been changed into brown coal and jet, 

 the black compact variety which frequently takes a polish. Some of 

 the pebbles and boulders contain the guide fossils of Silurian and 

 Devonian strata now outcropping in the Appalachians, thus showing 

 their origin. In most cases, however, the boulders are of quartz from 

 the many veins exposed on the Piedmont. In these fossiliferous 

 boulders the fossils occur as molds and casts^ the first referring to the 

 hollow imprint of the organism and the latter when this has been 

 filled subsequently with mineral material. 



ENTRANCE GATES 



The Harvard Street entrance to the Zoo (pi. 2) is now close at 

 hand with its gates, a favorite place of assembly and study for 

 visiting groups of university students. The stone pillars which sup- 

 port the gates, when rebuilt in 1932, were constructed of hand- 

 trimmed blocks of the three main rock types, selected as a visible 

 demonstration of the classification of rocks. Furthermore, slabs of 

 each type have been placed in both vertical and horizontal positions 

 in order to show the varying effects of weathering under these dif- 

 ferent conditions. The adverse effect upon the slabs placed vertically 

 interests the students particularly, for they take delight in prying 

 off slivers of rock which have been loosened at the surface by the 

 two processes of alternate heating and cooling, and freezing and 

 thawing {mechanical disintegration). Weathering by solution 

 {chemical decomposition) is also evident in many of the slabs where 

 in a few years' time the hard rock is softened by rain water which 

 dissolves away the cementing material or reduces the feldspar in the 

 granite to clay. Sedimentary strata are so poorly represented in 

 the Zoological Park that there is little opportunity to study them 

 in natural outcrop, but a few varieties are included in these pillars. 

 The capstone (6) is a sedimentary rock of continental deposits^ 

 mainly wind-blown sands, the grains rounded by abrasion and con- 

 solidated by cementation into solid cross-bedded layers of red to 

 brown sandstone. A marine sandstone made up of angular grains 

 deposited under the sea, where the buoyancy of the water prevents 



