640 THE JOURNAL OF GEOLOGY. 



I. a. Allothimorphic fragments, with the composition and form of 



unaltered clastic constituents. 

 b. Authimorphic fragments, with the composition of the clastic 



constituents unchanged, but the form altered. 

 II. a. Allothimorphic pseudomorphs, with the composition changed in a 



manner depending on the nature of the former substance, but 



with unchanged form ; dependent new products with old 



allothimorphic form. 

 b. Authimorphic pseudomorphs, with the composition changed as 



above, but with altered form ; dependent new products formed 



with new, authimorphic form. 

 III. Eleutheromorphic new products, with romposition and form 



altered ; true authimorphic products. 

 Under the division of authimorphic fragments two classes are dis- 

 tinguished ; those which have become adapted to the altered conditions 

 are designated as kamptomorphicj the others are called authiclastic. All 

 of the categories above tabulated are observed in the rocks of the 

 Murgthal. The first and second divisions are observed especially in 

 the quartz, feldspar, and mica, while the third is best seen in the 

 sericite and iron hydroxide masses. In the latter case, there is a varied 

 arrangement of the leaves and fibres. Where the new products are 

 radically arranged around the larger minerals, the structure is desig- 

 nated as eleutheromorphic-lenticular (flaserig). If these new formations 

 wind around the larger mineral components, the structure is called 

 mechanical-lenticular (flaserig). In the first case, the single leaves are 

 not deformed and they show no optical disturbance, while in the 

 second instance these leaves are bent. The former are analogous to 

 the links of a twisted chain, and the latter to the strands of a cable. 



The author compares with the altered sandstones of the Murgthal, 

 the curved lenticular gneiss of the Rhine valley. These rocks have 

 been strongly contrasted by all previous students, but Dr. Milch 

 regards the difference between them as one of degree rather then kind. 

 In both the original unaltered material was allothimorphic. The 

 present authimorphism has been produced by chemical and mechanical 

 means. The cause has usually been regarded as pressure alone. This 

 explains the mechanical alteration of the components, but the chemical 

 alteration is only to be explained by a transfer of substance through 

 the agency of water. Dislocation and faulting are not necessary to 

 produce these changes, for pressure and high temperature are sufficient. 



