STUDIES FOR STUDENTS. 84, 7 



oelow the underlying rock whence the earths were derived. As 

 shown in the figure, the upper surface of the rock is often so much 

 broken, and so much mingled with the more earthy subsoil, as to 

 make it very difficult to locate the plane of contact between them. 

 The true theory of the drift must explain its relation to the 

 rock beneath. Any hypothesis which fails to explain this rela- 

 tionship must be incomplete at the very least. Any hypothesis 

 with which this relationship is inconsistent, must be false. 



Significant Features of the Surface of the Rock 

 Underlying the Drift. 



Striation and planation of the bed rock. Besides the charac 

 teristics referred to in the last paragraph, the rock surface beneath 

 the drift, and especially beneath the unstratified drift, is frequently 

 found to be polished and striated. Sometimes it is polished 

 without being striated, but the two things usually go together. 

 While these features are found at many points throughout the 

 drift-covered area, and at all elevations at which drift occurs, 

 they are not found everywhere where there is drift, and are rarely 

 found beyond its limits. Where similar striae on bed rock are 

 found outside the limits of the great drift area, it is not with- 

 out significance that they occur in lesser areas of drift. In 

 North America, these lesser bodies of drift, with their striated 

 stones, and with striated bed rock beneath, are in the lofty moun- 

 tain regions of the west. The smoothings and the markings on 

 the bed rock beneath the drift are identical in kind with those 

 already noted as occurring on the bowlders of the drift. So exact 

 is the correspondence, that community of origin cannot be doubted. 

 The drift agencies, therefore, must have been capable of stria- 

 ting the rock beneath the drift, as well as the stony materials of 

 the drift itself. 



The striae on the bed rock beneath the drift are generally 

 approximately parallel in any given locality, and tolerably con- 

 stant in direction over considerable areas. In a general way, the 

 direction of striae corresponds with the direction in which the 

 drift has been transported. When large areas are studied, the 



