DEMONSTRATION MATERIAL IN GEOLOGY 65 



genious geologist will reconstruct the model, so that it will be more 

 serviceable and attractive. 



12. A piece of apparatus designed by W. H. Hobbs 7 to teach students 

 to interpret geologic maps by reproducing field conditions on a small 

 scale, which has been successfully used at the University of Michigan for 

 a number of years, is described in his "Earth Features." Similar appa- 

 ratus has been employed in the University of Illinois and probably else- 

 where. 



13. H. G. Turner, 8 of Lehigh University, has made a useful and inex- 

 pensive piece of simple apparatus for teaching dip and. strike. 



14. Stephen Taber, 9 of South Carolina, has constructed apparatus for 

 illustrating (1) the growth of crystals under pressure, (2) the growth of 

 fibrous veins, and (3) expansive pressure due to hydration. 



The apparatus described above will doubtless be of use for advanced 

 students in economic geology, but not for elementary classes. 



15. The sedimentary erosion apparatus used at the University of Wis- 

 consin is shown in figure 8. The following description of the apparatus 

 is by W. H. Twenhofel : 



"The apparatus consists of two galvanized iron tanks and a glass tank. The 

 larger galvanized iron tank is shown in the right background. It is open on 

 one end, and from this end a trough of varying width connects with the smaller 

 galvanized tank, the latter being shown in the left background. The glass 

 tank is shown in the right foreground, and it is connected with the main 

 trough by one considerably smaller. This trough is so arranged that it may 

 be closed as desired. The glass tank has a galvanized iron bottom and one end 

 is constructed of the same material. The two sides and the end into which 

 water flows are made of glass, the end being inclined. 



"Erosion in the large tank is brought about by means of a spray, which may 

 be directed downwards, as shown in the photograph, so as to produce the 

 effects of rapid erosion ; or directed upward, so as to produce the effect of a 

 gentle rain. 



"The larger tank is mounted on a tilting-top table, which permits any corner 

 or any side to be raised or lowered at will, giving the effect of a tilting or 

 warping land. The slope of the trough is also adjustable. The material in 

 the tank may be varied to meet the requirements. 



"The smaller galvanized iron tank has three drains on the far end — one at 

 the bottom, one about 6 inches from the bottom, and one a foot from the bot- 

 tom. This permits adjustment of the water to any level, so that the effects of 



7 W. H. Hobbs : Apparatus for instruction in geography and structural geology : In- 

 terpretation of geological maps. School Science and Mathematics, vol. 0, 1900. pp. 044- 

 G53 ; also, Earth Features, p. 64, figures 47 and 4S. 



8 H. G. Turner: Laboratory determinations of dip and strike. Science, vol. lv. 1021. 

 no. 1411, pp. 53-54. 



Stephen Taber : American Journal of Science, June, 1010. vol. xli. pp. 532-536, fig- 

 ures 1, 2, and 3. Am. Inst. Mining Engineers Bulletin, no. 110. pp. 1973-199S. 



V— Bull. Geol. Soc. Am., Vol. 33, 1921 



