PETROGRAPHIC TERMS FOR FIELD USE 



ALBERT JOHANNSEN 

 The University of Chicago 



Velut aegri somnia, vanae 



Finguntur species, ut nee pes nee caput uni 



Reddatur formae. 



— Horace, De arte poetica, 7. 



Horace wrote this stanza with no idea of how aptly it would 

 apply to the megascopic classifications of rocks in use at the 

 beginning of the twentieth century. When the estimable authors 

 of the Quantitative System appended a field classification to their 

 system, they recognized the impossibility of accurately deter- 

 mining rocks megascopically, and said: 



It is obvious that a considerable part of the system of classification and 

 nomenclature here proposed can only be applied upon microscopical or chemi- 

 cal investigation For these reasons .... we are convinced of the 



need of general petrographical terms, which will be serviceable in the field work 

 of the petrologist, and which will be of use to the general geologist and to 

 those who may not be able to carry on microscopical and chemical investi- 

 gations. 



The authors proposed to select, from the terms originally given 

 by the geologists of the past, certain rock names and to give to 

 them their original significance "so far as possible, with only such 

 modifications as a somewhat more systematic treatment of the 

 matter may suggest." The suggestion was truly commendable, 

 for the need of a field classification was seriously felt. Unfortu- 

 nately, perhaps, we are not living in the old days. The former rock 

 terms have, in the course of time, acquired new meanings, and 

 an attempt to revive the old ones produces much confusion. 



Ut sylvae foliis pronos mutantur in annos, 

 Prima cadunt; ita verborum vetus interit aetas, 

 Et juvenum ritu no rent modo nata vigentque. 



— Horace, op. cit., 60. 



And so it is with the old petrographic terms. 



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