4o6 THOMAS L. WATSON 



direction of strike of most of them is in the same quadrants as the 

 strike of the joints. Furthermore, in nearly every quarry exposing 

 dikes of basic igneous rocks the strike of the dikes and that of one set 

 of joints were coincident, probably indicating that for the areas men- 

 tioned the jointing has exercised some influence in the cutting direc- 

 tion of the dikes. 



Not only is this true of the dikes penetrating the granites, but it 

 is equally true of the Triassic sandstone belt, where coincidence in 

 strike of the diabase dikes and that of jointing in the sandstone is 

 very strikingly shown. 



Careful measurements in the strike of the joints and the dikes were 

 made in the numerous openings over the sandstone belt, which can 

 be summarized as follows:^ Variation in major jointing is from 

 N.i5°-6o° W. to N. 20°-6o° E., with minor sets noted in some open- 

 ings striking N.-S. and E.-W. Likewise, variation in strike of the 

 dikes is from N. 2o°-6o° W. to N. 20° E., with a few striking N.-S. 

 In every opening the strike of the dikes and that of the joints for a 

 given direction was found to be coincident. 



Whether this will apply in general to those dikes beyond the limits 

 of the fresh rock exposures it is not possible to say at present, as the 

 jointing is entirely obscured by the deep residual decay covering the 

 fresh rocks. 



AGE-RELATIONS OF THE DIKES OF BASIC IGNEOUS ROCKS. 



As noted above in the basic dikes penetrating the crystalline rocks, 

 strongly contrasted structural differences in the' dike rocks obtain. 

 Many of them are entirely massive and unaltered, bearing little or no 

 evidence of pressure-metamorphism, while a large proportion of them 

 are completely schistose, and are otherwise mashed and closely 

 jointed. The ferromagnesian constituent in the latter is usually 

 partially or completely altered. Both classes of the dikes often pene- 

 trate original massive igneous rocks that are now more or less schistose 

 in structure. These facts afford a strong and sufficient basis for 

 regarding the basic rocks of the dikes of different periods of intru- 

 sion, and therefore, of different age. The massive dikes penetrating 

 the more or less schistose rocks must postdate in age the period of 



* Data kindly furnished by Mr. F. B. Laney. 



