tyndall's experiment. S3 



It might seem as if the varying directions of pressure detected in Tyn- 

 dall's experiment were geologically unimportant. Granting that the 

 vertical, uniform pressure at first applied to the wax is conically resolved, 

 does it not follow that in orogenic movements also a similar resolution 

 occurs ; so that, after all, slaty cleavage is due to a pressure originally 

 uniformly distributed and perpendicular to the cleavage? This query 

 must receive a negative reply. 



The reason why the pressure in the experiment is resolved into a 

 conical system of forces is that the hodies between which the wax is 

 squeezed do not themselves yield sensibly. Thus horizontal relative 

 motion attended by friction is brought about. Were these bodies as 

 soft as the wax, they too would extend, laterally and the pressure 

 would remain uniformly distributed. It would also produce no slaty 

 cleavage. 



In orogenic movements there is seldom any diversity between the 

 resistance of adjoining rock masses approaching the difference between 

 plates of glass and warm wax. Among rocks, therefore, a direct pressure 

 will, as a rule, be distributed with an approach to uniformity, and there 

 will be little or no relative motion between adjoining rock masses in 

 directions perpendicular to the pressure. Hence, also, important masses 

 of slate will not be produced in this way. 



Perhaps no combination is entirely wanting in mechanical geology. 

 In artificial cuttings, clay beds underlying harder materials have been 

 known to be squeezed out laterally, and these masses must have been 

 affected like the wax in Tyndall's experiment ; but this case scarcely 

 forms an important exception. 



In most cases of the geological occurrence of slate there is little direct 

 evidence of the mode of formation, and it is for this reason that the ex- 

 periments are of so much value. Sometimes, however, the method of 

 formation of natural slate is clear. I refer especially to the slaty selvedges 

 which are not infrequently seen bounding small faults in granite and 

 which have been mentioned under the head of secondary action on 

 ruptured rock. No geologist can doubt that these selvedges are produced 

 by the inclined pressure attending faulting, and it is manifest that the 

 distortion of an elementary cube would be exactly that which so con- 

 stantly accompanies the artificial production of slaty cleavage. Thus, in 

 some cases at least, natural slate is produced by the same means which 

 are employed in producing artificial slate. 



Behavior of included Grit Beds and Fossils. — The theory that slate is 

 produced by a uniformly distributed pressure perpendicular to the planes 

 of cleavage, such as it has been usual to suppose existed in Tyndall's 

 experiment, implies that the strain ellipsoid is an oblate figure of revolu- 



