FOLDING AND THICKNESS OF RANDVILLE DOLOMITE. 433 



for the dam. The light-blue and pearly- white layers of the beautiful mar- 

 ble here seen are thrown into a series of unsymmetrical folds. The western 

 sides of the little anticlinals are short and ovei'turned, while the eastern 

 sides are long and gently inclined. Evidently, if the same system of sec- 

 ondary folding holds throughout the entire thickness of the formation, sur- 

 face observations would show everywhere eastward dips at variable angles, 

 dependent upon the portion of the fold which happened to constitute the 

 particular outcrop, and gentle dips would be more abundant than steep dips. 

 This would completely explain the observed variations. 



Similar variations and lack of regular sequence in the dips are found 

 in the southern river section. Five good observations range between 20° 

 and 58°, all eastward, but none of the exposures is sufficiently extensive to 

 show minor folds. The mean of these observations is about 40°. 



The surface width of the dolomite zone on each section is a little less 

 than 3,000 feet, assuming that a fair proportion of the covered zones on 

 each side is underlain by the same formation. If the average observed dip 

 is taken to represent the average dip of the rock, the thickness in each 

 case would be a little over 1,900 feet. This is probably too great, and is 

 certainly too great if the same kind of internal crumpling visible in parts 

 of the upper river section is characteristic of the formation throughout. 

 The average dip evidently would more nearly be represented by the dips 

 of the long eastern limbs of the little anticlines. Assuming that these are 

 less than the mean, we find the average of the dips below 40° to be 30° for 

 each section. This gives a thickness of about 1,500 feet, which still is per- 

 haps beyond the truth, but is probably much nearer it than the first value. 



It is interesting to compare this result with the thickness of 500-1,000 

 feet obtained on the two Felch Mountain sections. A part of the increase 

 is probably due, as already explained, to the earlier beginning of limestone 

 deposition in the Michigamme area. But an important part of it is prob- 

 abl}^ not depositional at all, but is the result of plications. The whole 

 series here is but gently tilted as compared with the walls of the Felch 

 Mountain trough, and hence the strong horizontal pressures have acted 

 in a direction but slightly inclined to the bedding. The result has been 

 the secondary crumpling within the formation which must contribute in an 

 important degree to its present apparent thickness. 



In the scattered outcrops of the Michigamme Mountain area the 



MON XXXVI 28 



