MOUNT GREYLOCK. 



155 



or wedge-shaped masses. This is the typical slip cleavage. The minute 

 structure at the contact, as seen in a microscopic section, corresponds to 

 that represented in the diagram, Fig. 55. The inference from such facts 

 is that while conformable contacts are all-important in determining strati- 

 graphic relations in a metamorphic region they may be entirely misleading 

 unless it can be shown that the foliations which conform to the plane of junc- 

 tion between both rocks are indeed stratification foliation. 1 



CORRELATION OF CLEAVAGE AND STRATIFICATION. 



The facts adduced naturally raise the question as to the general cor- 

 relation of cleavage and stratification. The relations of the strikes of the 

 two foliations have already been explained under Case vn. As to the dip 



Fig. 5G. — Thin section of schist from locality 115, on the Bald Mountain spur, enlarged H diameters, showing the paral 

 lelism between the cleavage planes and the axial planes of the plications. From a photograph. 



of the two foliations: The range of the difference in angle of dip between 

 cleavage foliation and stratification foliation in sixty-three observations was 

 found to be from 10° to 120° ; 2 the average difference 62°, 30'. The abso- 

 lute dip of cleavage in ninety-six observations, in which the dip of stratifi- 

 cation foliation was also observed, ranged from 10° to 90°, averaging about 

 45°; leaving out eleven extreme cases the range was from 25° to 75°, and 

 the average 44°. 3 The direction of the dip of the primary cleavage in one 

 hundred and nineteen localities, in which that of the stratification was also 

 determined, was distributed as follows: ninety-two localities east or north- 

 east, twelve west, four vertical, one south. The southerly dip occurs at the 



1 Compare J. D. Dana, Taconic rooks and stratigraphy. Am. Jour. Sci., Ser. Ill, vol. 33, May, 1887, 

 p. 398, in which the possibility of such cases as this is overlooked. 



'When the difference is over 90° the direction of the two dips is opposite. 



'Where cleavage is horizontal and stratification nearly or quite vertical, as is sometimes the case 

 in the Berkshire county schist, there have probably been two uplifts. 



