SUMMARY 299 



crystalline texture. When solidified as sheets or dikes, the igneous 

 rocks may be folded, faulted, cleaved, or metamorphosed like the 

 strata, and when a region has been long and repeatedly subjected 

 to compression, its structure may become excessively complex, 

 and the metamorphosis of its rocks so complete that not even the 

 most careful examination will suffice to distinguish those rocks 

 which were originally sedimentary from those which were igneous. 

 Our study has taught us that many of these processes go on 

 deep within the earth's crust, and hence cannot be directly ob- 

 served, but must be inferred from their results. Very encouraging 

 progress has already been made in this work, but much remains to 

 be done before our knowledge of structure and its full meaning 

 shall be even approximately complete. 



