524 FRANK D. ADAMS 



to powder when the alum was dissolved away. A photograph of one 

 of the columns as it appeared when freed from the alum is shown in 

 Plate VI, Fig, c, but as in this particular case the movement was not 

 so great as in others, the gneissic structure referred to is not well seen. 

 When examined under the microscope, both quartz and orthoclase 

 show well-marked strain shadows, but even a very careful examina- 

 tion under a high power between crossed Nicols fails to show with 

 certainty whether the shadows in question are due to an actual 

 bending of the mineral or to a fracture of the mineral with a slight 

 shifting along an infinite number of ultra-miscroscopic cracks. 



In several cases, however, where the conditions for observation 

 were very favorable, no signs of such cracks could be detected and 

 the mineral seemed to have undergone an actual twisting. 



The biotite individuals had been very distinctly bent and twisted. 



The rock displays a remarkably perfect cataclastic structure along 

 certain lines or streaks where the quartz and orthoclase are repre- 

 sented by larger fragments, which, however, are mingled with those 

 of smaller size, as well as with others which pass into almost ultra- 

 microscopic dimensions. 



SUMMARY 



1. Under the differential pressures developed by this method of 

 experimentation, that is by employing Kick's process, using fused 

 alum or the other embedding materials employed, and tubes of copper 

 with walls of from 0.125 to 0.25 inch (3.175 to 6.38 mm.) in thick- 

 ness, minerals which have a hardness of 5 or under (Mohs's scale), 

 show distinct plastic deformation, this deformation being less pro- 

 nounced in the case of the harder minerals. 



2. The minerals above 5 on the Scale of Hardness, while not 

 presenting any marked change in shape, in some cases show evidences 

 of internal movement. Thus a perfect basal twinning is developed 

 in diopside, similar to that so often seen in specimens of this mineral 

 from the crystalline limestones of the Grenville series. 



3. In the case of very hard minerals, no evidence of plastic flow 

 was discernible; their structure was broken down and they were 

 reduced to powder. 



