Crosby.] 



246 



[Dec. 16, 



set the intervals between successive joints are remarkably regular. 

 These intervals were measured in series of ten each, with the re- 

 sults tabulated below, omitting fractions : 



I. 



II. 



III. 



IV. 



V. 



Inches. 



Inches. 



Inches. 



Inches. 



Inches 



11 



9 



10 



12 



11 



11 



8 



10 



11 



12 



11 



12 



10 



11 



11 



12 



10 



12 



10 



10 



12 



8 



10 



9 



12 



13 



11 



9 



10 



13 



13 



10 



8 



13 



11 



12 



11 



10 



14 



10 



9 



13 



11 



13 



12 



14 



11 



9 



15 



11 



Average 11.8 10.5 9.9 11.8 11.3 



The general average of the fifty measurements is eleven inches, 

 which differs but little from the average for each series. Five 

 other series of measurements were made at a point one-half 

 mile distant, where the jointing is less regular. The average dis- 

 tances are greater and more variable ; but this is partly owing 

 to the cliff being more irregular and crossing the joint-planes 

 obliquely. The joints in the system parallel with the cliff are 

 naturally not so well exposed for observation ; but the intervals 

 between adjacent joints seems to vary but little from two feet. 

 This system greatly facilitates the erosion of the cliff. The incli- 

 nation of the joints is so great that when the waves acting on the 

 base of the cliff have cut through the thickness of one joint layer 

 the whole mass, two feet thick and perhaps fifty or one hundred 

 feet high, slips down bodily, leaving a fresh joint surface to form 

 the face of the cliff. The erosion of the second layer can not 

 begin until the debris of the first is removed, and thus the surface 

 of the cliff' is kept straight and parallel with the joint planes. 

 The main channel of the river is half a mile to a mile from the base 

 of the cliff; and the intervening shallow water increases in depth 

 very gradually, covering a smooth, hard floor or platform, which 

 has been formed by the backward erosion of the cliff. 



That the joint structure of this formation can not be explained 



