OHIO YALLEY-BALCOXES LINE OF DISTURBAXCE 547 



to New York. I do not think tliej' are along connected lines of disturbance, 

 but follow the local lines of weakness. I know that near Ithaca they are 

 parallel with the master joints." 



The map presented (figure 2) will show the relation of the Balcones- 

 Arkansas-Ohio Valley line of weakness and the igneous exposures to the 

 adjacent mountain masses. 



Another thought comes to mind. It is possible that the New Madrid 

 earthquake, the center of which was very near, if, indeed, not in direct 

 line with this zone of weakness, might have been caused by slipping along 

 this fault zone. 



Scattered Occurrences of Crystallixes 

 general statement 



In addition to the Precambrian crystalline rocks exposed in the moun- 

 tain uplifts, and the various post-Cambrian intrusives occurring either 

 in the Arkansas-Texas zone of weakness or in the region near the base 

 of the Eockies, there are on the plains several localities where crystalline 

 rocks of various sorts have been brought to the surface. One thinks at 

 once of the Sioux Falls quartzite and associated rocks, which outcrop 

 over considerable areas in Minnesota and South Dakota and w^hich have 

 been found in many wells in regions adjacent to the outcrops of these 

 crystallines. 



There are at least five places in the plains where very small and incon- 

 spicuous exposures of crystalline rocks occur. These occurrences, which 

 will be discussed in brief detail, will serve to illustrate that Kemp's pro- 

 nouncement that "we are never safe from igneous rocks, no matter how 

 innocent flat strata look from the surface." 



8PAVINAW CREEK, OKLAHOMA 



For many years we have known that an exposure of granite occurred 

 along Spavinaw Creek, in Mayes County, eastern Oklahoma. This rock 

 was first mentioned by D. D. Owen, and later visited by Drake,^ who 

 referred to it as a granite dike. Snider^^ and others who have published 

 on this locality have generally followed Drake in considering it a dike. 

 The exposures, consisting of red granite which has the general appear- 

 ance of the granite of the Wichita Mountains, are about 1,200 feet long 

 and 50 feet wide. 



Fritz Aurin, who recently studied this exposure, says^^ 



"It is not an igneous dike, as is generally thought, but is a granite peak of 

 the old Precambrian basement. Overlying the granite is a siliceous, dolomitic 



