Pogue, Jr. — Geology and Structure of Volcanic Rocks. 235 



In the cross-section given no minor crumplings are indicated 

 on the major folds. Such undoubtedly exist, but they are 

 omitted both for the sake of simplicity and because their 

 nature is not known. The thickness of those beds whose 

 cross-section is in no place exposed upon the surface is entirely 

 hypothetical. If it be attempted to trace out each formation 

 as it is successively brought to the surface by the folding, it 

 will be found that the beds do not always match on the 

 opposite limbs of the folds. This is understood, if it be 

 accepted that the original horizontal extent of the formations 

 consisted of a complicated interfingering of beds and lenses, 

 and was not a succession of regular beds of the same thickness 

 thoughout their lengths. In such a case folding w T ould not 

 necessarily repeat similar beds on corresponding part of folds. 

 Along Flat Swamp Ridge, however, this explanation seems 

 inadequate, inasmuch as beds of great thickness must pinch 

 out with extreme rapidity in order to avoid repetition. To 

 obviate this difficulty, an overthrust fault has been introduced 

 as stated, as a simpler explanation. The arc-like surface trace 

 of this supposed fault-plane renders it probably an overthrust 

 consequent upon the northwest compressive force of folding. 

 The fault-plane may itself be folded, depending upon whether 

 the slip occurred at the beginning or near the end of the 

 period of folding. The latter conception is preferable, because 

 simpler. 



Alternate hypotheses. — Although the interpretation of struc- 

 ture given appears to best fit the facts, some alternate hypothe- 

 sis, especially in regard to the subordinate features, may be 

 mentioned. 



As suggested in previous paragraphs, the entire structure 

 may be explained without the use of faults. 



A portion of the rocks may have been brought into the 

 region by an overthrust fault ; and these subsequently inter- 

 folded with the regional rocks. 



The region may possibly represent a series of isoclinal folds, 

 with parallel limbs. 



That the region may not be folded and represents a very 

 thick deposit, which has been tilted and whose edge is now 

 cut across by the plane of erosion, seems hardly a possibility. 



Discussion of geologic history. 



The geologic history of the Cid district may be considered as 

 having its beginning during a period of volcanic activity of 

 long duration. During this time there were innumerable 

 alternations between quiet upwellings of lava, forming surface 

 flows; explosive activity on an enormous scale, piling up 



