310 Cok EELPEH. 
Whether with more intense anamorphic conditions the sedimen- 
tary rocks actually pass into igneous rocks by subcrustal fusion or 
some analogous process, is yet a question which it is difficult to decide 
by direct observation, and which is partly a matter of definition. 
The burden of proof rests on those who hold that actual evidences 
of the change may be observed. It is easy to conceive that slightly 
assorted sediments containing essentially the minerals of the original 
igneous rocks may take on the characteristics of igneous rocks under 
these conditions; but, in proportion as the ultimate products of 
katamorphism have been developed and there is corresponding seg- 
regation of the simpler compounds, it is difficult to tell how these 
may again be brought together on any large scale to reproduce the 
proper mineralogical and chemical combinations of the igneous 
rocks. While silicates are developed, the main mass retains its 
simpler chemical characteristics and can be often distinguished 
from an igneous rock. If, as held by Van Hise, the processes of 
katamorphism and anamorphism both take place with a liberation 
or running-down of energy when both chemical and physical factors 
are considered, the completion of the metamorphic cycle on any 
broad scale seems doubtful. | 
It is further not clear whether the development of schists and 
gneisses from igneous rocks is predominantly katamorphic or ana- 
morphic for the minerals of the igneous rock. In some observed cases 
the change is clearly katamorphic, such as the common development 
of hornblende from augite, of the alteration of a basic feldspar to 
a more acid feldspar with separation of clay and calcite, or the altera- 
tion of a potash feldspar to kaolin and sericite, although, as already 
indicated, one of the resulting minerals may be fully as complex as 
the original minerals. It is also equally certain that the observed 
development of heavy, anhydrous minerals, such as garnet, in the 
schistose igneous rocks means anamorphism. 
The cycle—The sequence of destructive and reconstructive 
changes above outlined constitutes a partial metamorphic cycle. If 
the slates, schists, or gneisses ever take on the characteristics of 
igneous rocks, through fusion and mixing, the cycle is complete. A 
necessary consequence of the cycle theory is that in the same zone 
changes may be both destructive and constructive—destructive to 
