48o 



PRINCIPLES OF STRATIGRAPHY 



purity or "ash" being primary, i. e., due to the non-burnable mineral 

 matter originally a part of the plants, or secondary, due to the ad- 

 mixture of foreign material, either mechanical sediment or chemi- 

 cal precipitate. Though rare, absolutely pure sapropelitic deposits 

 occur. According to the stage of consolidation and age, these have 

 received distinct names, as shown in the first columns of the sub- 

 joined table after Potonie (32:55). 



In the purest state largely or almost entirely 

 biogenic and therefore aquatic — autochtho- 

 nous formations, i- e., produced through 

 autochthonous sedimentation. 



Allochthonous, with ref- 

 erence to the lutaceous 

 or finely arenaceous con- 

 stituents, but autoch- 

 thonous, with reference 

 to the organic constitu- 

 ents, therefore produced 

 by allochthonous and 

 autochthonoiils sedimen- 

 tation. 



Petroleum. Among the fossil sapropelytic substances petroleum 

 takes the first rank. It is, however, to be regarded as a diagenetic 



