DEFINITION OF TERM APO. 777 



all the textures of glass, but which has been completely devitrified, apo- 

 rhyolite. In her original definition she restricted the term to this single 

 alteration, but later she enlarged the meaning 11 so as to include not only 

 devitrified glasses but alterations of all rock in which the original textures 

 and structures are preserved. 



The prefix apo thus used discriminates rocks which have been meta- 

 morphosed under mass-static conditions and retain their original textures 

 and structures from those which have been metamorphosed under mass- 

 dynamic conditions, so as to destroy or partly destroy the original textures 

 and structures and to produce slaty, schistose, or gneissic structures. A 

 rock to which apo is prefixed may differ greatly from the original rock in 

 both chemical and mineral composition. 



The term epi was proposed by GiimbeP as a prefix for rocks which have 

 undergone a change in mineral composition, and which by this change have 

 come to have the same mineral composition as another rock. Thus a dia- 

 base the pyroxene of which has changed to amphibole, and which, therefore, 

 has the mineralogical composition of diorite, he calls an epidiorite. The 

 term diabase itself was originally applied to an altered dolerite, which has 

 as an important constituent secondary chlorite. Applying the method which 

 is proposed to be here followed, the rock which Giimbel calls epidiorite 

 would be called apodolerite. This name is much more satisfactory than 

 that of Giimbel, since it gives the original nature of the rock, and tells 

 that a part or all of its minerals have altered, but that it retains its original 

 textures and structures. Therefore the name apodolerite gives a better 

 understanding of the history and relations of the rock than does the name 

 epidiorite. 



In a manner similar to the above apo may be pi'efixed to any rock 

 altered under mass-static conditions, when it is desired to call attention to 

 the rock from which it was derived. Thus quartzite is an aposandstone; 

 graywacke is an apog-rit, etc. The term apo thus used supplements the 

 terms schist and gneiss, its usage being structural in a negative sense — 

 that is, applied to metamorphic rocks in which the textures and structures 

 have not changed. 



«Bascom, Florence, Volcanics of Neponset Valley, Massachusetts: Bull. Geol. Soc. America, 

 vol. 11, 1900, pp. 121-122. 



b Giimbel, K. Wilhelm von, Geoiogie von Bayern, Kassel, 1888, vol. 1, p. 125. 



