1885.] 245 [Crosby. 



as a rule, the angle of intersection of the systems of joints is 

 large, it is occasionally very small, angles as low as 5° having 

 been observed. These exceptional examples seem to vitiate the 

 preceding explanation of the normal intersections, and Mr. Gil- 

 bert does not reconcile the two features. It seems to me, however, 

 that they are easil} 7 harmonized, if we make the not unreasonable 

 assumption that while oblique vibrations of low velocity would be 

 relieved by slipping along the preexisting joint planes, vibrations 

 of high velocity would be so nearly instantaneous as to produce 

 fractures normal to their direction before the inertia of the rock in 

 the direction of the preexisting joints could be overcome. 



Although I have not met any insuperable objections to the earth- 

 quake theoiy, several striking confirmations of it have come under 

 my notice. One of these in particular seems to merit description. 



Among the most prominent features of the lower Potomac River 

 are the Nomini Cliffs which form the west shore for about six miles 

 in the eastern part of Westmoreland Count}'', Virginia. 1 They are 

 from one hundred to two hundred feet high and consist entirely of 

 Miocene clays and sands, which are nearly horizontal, dipping 

 gently to the east. In the southern and central portions of the 

 cliff the beds are considerably varied in color and composition and 

 some layers are highly fossiliferous ; but the northern part is re- 

 markably homogeneous, consisting from top to bottom of a fine 

 grayish clay in which the stratification is scarcely distinguishable. 

 The slope of the cliffs varies from 50° to 75° and is certainly very 

 steep, considering that the materials are entirely unconsolidated, 

 with the exception of certain limited layers of ferruginous sand 

 and gravel. The face of the cliff is clear and bare in the north 

 part, but farther south it is more or less obscured by a sloping 

 talus. A distinct joint structure is more or less noticeable at all 

 points ; but in the northern section, owing, probably, in part to 

 the more perfect exposure of the undisturbed strata, and partly to 

 their greater homogeneity, the jointing becomes a very conspicuous 

 and remarkable feature of the cliff. There are two systems of 

 joints. One of these is sensibly vertical and at right angles to the 

 general trend of the cliff; while the other is approximately parallel 

 with the face of the cliff. 



The joint planes are very straight and continuous, and in each 



*The observations upon which the following description is based were made chiefly 

 by my father, F. W. Crosby. 



