EFFECTS OF INCLINED PRESSURE. 55 



microlites, most of which are parallel to the banding. Those microlites 

 are no doubt of greater density than the glass, but, on account of the 

 viscosity of the melted glass and the enormous surface per unit volume 

 which the microscopic prisms expose, they cannot be supposed to attain 

 their actual arrangement as a result of gravity like the mica scales in 

 sandstone. On the other hand, if one supposes an irregular orientation 

 of the microlites in the glass, and that tangential motion has been set up 

 between adjacent layers of the viscous mass, every microlite standing 

 across the direction of relative motion would be swung into the line of 

 relative motion by the opposite traction exerted on its two ends by the 

 moving layers. It appears to me, therefore, that " rhyolitic structure " 

 indicates "shearing motion " or, as I have called it, scission in the direc- 

 tion of the banding * 



Inclined Pressure and yielding Medium. — An inclined pressure acting on 

 a tabular mass of rock is equivalent to a direct pressure and a tangential 

 force. This last, with the resistance necessary to keep the center of 

 inertia of the rock at rest, forms a couple. If the rock is surrounded by 

 masses of comparatively feeble resistance, it will then rotate until the 

 couple is exactly balanced 1 >y the resistance to rotation. The rock is thus 

 subjected to the action of a simple pressure and two balanced couples, 

 constituting a simple shear, neither of the axes of which coincides with 

 the line of pressure. 



As has been shown above, the strain produced by a pressure and a 

 shearing stress is a rotational one, the amount of rotation, however, being 

 small as compared with that involved in some other strains. One of the 

 directions of maximum tangential strain will therefore sweep over a 

 greater range of material particles than the other, or will affect a given 

 set of particles for a shorter time. That set of planes of maximum strain 

 which shifts its position more rapidly will encounter greater resistance 

 from viscosity and will produce the smaller effect. 



If the mass is strained beyond the elastic limit, but not to the point of 

 rupture, a schistose structure will result , but one set of schistose partings 

 will be confined to a somewhat smaller angle than the other and the 

 more pronounced partings will be associated with the smaller angle. 



If the pressure is intense enough to produce rupture, fracture will take 

 place chiefly along the partings which have the smaller range. 



The axes of the strain ellipsoid will bisect the angles which the last 

 schistose partings make with one another, and the minor axis of the 



* The above discussion is incomplete. A full treatment would ot cour ■ i ign a definil i value to 

 the couple which resists the tilting of a disc moving in a fluid. The reader will find the subject 

 more fully developed in Thomson and Tail, Nat. Phil., sections 320-325, with interesting instances. 

 That discussion is decidedly difficult, while the main points in which geologists are inl iresl ■'! 

 seem tu be adequately demonstrated by the exceedingly elementary method here presented. 



IX— Bull. Gkol. Soc. A.m., Vol. 4, 1892. 



