WRIGHT: A GEOLOGICAL PROTRACTOR 5 



y = ax; pv = c; y = ax 2 ; y % = ax; y = a*, 



or log y = x log a; 

 y = x Q , or log y = n log x. 

 sin i = n- sin r; D = sin A • sin A 1 ; tan 2 V = ^; 

 cot A = sin B ■ sin C; tan A = sin 5 • cot C; 



cos A = cos 5 • cos C; 

 sin A = sin B • sin C; tan A = sin B • tan C. 

 This list is by no means complete but it serves to indicate the 

 variety of equations which are of the form A = B- C and which 

 can therefore be solved graphically with a fair degree of accuracy. 



GEOLOGY. — A geological protractor. F. E. Wright, Geo- 

 physical Laboratory. 



In geological field and map work a protractor is commonly 

 used for plotting angles of strike and dip. For the drawing of a 

 vertical cross section it is also desirable to determine the line 

 of slope of any given bed on the section. Heretofore this has 

 been accomplished either by graphical methods or by compu- 

 tation or by use of a graphical computation chart. 1 It is pos- 

 sible, however, by slightly modifying the protractor, to combine 

 with it a slope angle computer such that the apparent dip of a 

 bed can be read off directly for any angle of dip of stratum 

 and for any azimuth of vertical section. 



The principles on which the slope computer is based are pre- 

 sented in detail in the foregoing paper. The equation to be 

 solved is of the form tan C = sin B ■ tan A in which A is the true 

 angle of dip, B the direction angle between the line of strike of 

 the bed and that of the vertical section, and C the apparent dip 

 angle on the vertical section. In solving this equation by the 

 graphical method here proposed it is important to note that, 



1 A chart of this nature was first described by W. G. Woolnough, Proc. Austra- 

 lasian Association for the Advancement of Science, 1909: 244-249. Practically 

 the same chart was published later by D. F. Hewett without knowledge, however, 

 of Woolnough's chart. Economic Geology, 7: 190. 1912; and still later by H. 

 Bancroft, Bull. Am. Inst. Mining Engineers, July 1914, p. 1769. A straight line 

 chart was first prepared by the writer. Journ. Wash. Acad. Sci., 4:440-444. 

 1914. 



