Jan., 1919. Annual Report of the Director. 247 



semi-tropical representative plants is an excellent one. The Section of 

 Plant Reproduction will remain in Florida until April or early May. 



In the Department of Geology about one hundred and twenty-five 

 specimens were added to the Chalmers Crystal collection and the whole 

 series re-installed, the additions making it necessary to move part of the 

 collection to another case. The specimens are all mounted in correct 

 crystallographic positions on individual mahogany stands and have 

 separate individual labels. In the arrangement of the collection as 

 exhibited, specimens illustrating the six crystallographic groups are 

 first shown in order and subsequent to these, specimens of twin crystals, 

 crystal groupings, crystal inclusions, crystal distortions, etc. The work 

 begun last year of repolishing and re-etching the sections of iron mete- 

 orites, has been continued, thirty-six specimens having been thus treated 

 during the year. These specimens have been re-installed as fast as 

 the work upon them has been completed. Several relief maps that had 

 been exhibited in the Court were removed and packed, while others 

 were re-installed. The large painting of a Montana glacier presented by 

 the Great Northern Railway was installed in this court in connection 

 with large specimens showing glaciated surfaces. A model of the Natural 

 Bridge of Virginia, based on the accurate survey and studies in the 

 field made by the Assistant Curator last year, has been executed by the 

 Assistant Curator and also placed on exhibition in the West Court. The 

 model is five feet six inches long, three feet three inches wide, and two 

 feet ten inches high, thus being of a size which is as large as will fit into 

 the ordinary type of case. The scale is ten feet to the inch. This scale, 

 without producing a model of unwieldy size, is large enough to show 

 minor details distinctly. In this model an attempt has been made to 

 simulate nature as closely as possible and to avoid the conventionalized 

 and generalized systems of representation frequently employed in 

 geological modeling. Also the vertical and horizontal scales are the 

 same, thus avoiding the distortion which is also frequently employed 

 in geological models. The model represents a length of 660 feet of the 

 gorge of Cedar Creek in Rockbridge County, Virginia, with the well- 

 known Natural Bridge of limestone spanning it at an elevation of 215 

 feet above the water. The stream bed and the vertical cliffs are of lime- 

 stone, with coatings of bright-colored clays. The higher levels and the 

 more sloping portions of the canon walls are densely wooded. The basis 

 of the model is reinforced concrete. The limestone surfaces and cliffs 

 are modeled, the concrete mixtures at the surface being varied to meet 

 the changing requirements of the rock texture. The strongly colored 

 surfaces of the limestone have been reproduced in the model by perma- 

 nent pigments which have been introduced into the pores of the con- 



