346 A SYMPOSIUM 



represented by the lithologic unit which may be mapped 

 on a given scale and the lithologic unit itself. 



Episodes for time, Lenses and Lentils or Stages for rocks. — 

 Terms applied to local lithologic variations or limited 

 rock masses which for purposes of discussion need to be 

 defined, but are not of sufficient consequence to justify the dis- 

 tinction of a separate name. 



I may briefly state my reasons for the above suggestions as 

 follows : (i) I associate era and system because the classification 

 is based on the broadest natural facts and is therefore systematic. 

 (2) I associate group with period because both terms appear to 

 me less precise and adapted to the arbitrary character of the 

 unit thus classified. The division of time according to a scale 

 of periods appears to me equivalent to the division of a column 

 of mercury according to a scale of degrees to indicate tempera- 

 ture ; whether the result be expressed in the arbitrary terms of 

 Fahrenheit or Centigrade the fact remains unmodified. The 

 simplest scale which will satisfy the needs of world-wide geology 

 is to be preferred. (3) I associate age and series as both of 

 them indicate a consistent logical sequence of events having 

 their beginning and rounding out to an end, as in history we 

 have the Elizabethan age and the series of events which character- 

 ize it. Age and series are natural divisions as distinguished 

 from period and group, which are terms of the arbitrary scale. 

 (4) I associate epoch and formation probably rather through 

 custom than for any special reason, but I prefer epoch as a time 

 designation to stage because the latter has a more concrete 

 significance and might with equal aptness be applied to a sub- 

 division of rocks. Indeed, if a geologist named to me the 

 Medina stage, I should understand that the sandstone was 

 referred to rather than the time for which the sandstone stands. 

 Furthermore a stage appears to me to represent a temporary or 

 very brief condition and to correspond in time with the term 

 episode. The necessity for the fifth subdivision, corresponding 

 to episode, frequently arises in detailed discussion, and is a 

 means of avoiding complexity of nomenclature. Thus if a con- 



