234 J. F. XEWSOM — CLASTIC DIKES 



bed of sandstone in the shale, the dip of which is 13 degrees north 80 

 degrees east. 



Six hundred feet below 16 a 2-inch intrusion (dike 17) is exposed on 

 the east side of the creek, and 350 feet still farther downstream 18 is 

 exposed. 



The dike at 18 (figure 1, plate 23) is exposed for about 120 feet along 

 the bluff at the east side of the creek. This intrusion is unlike those 

 seen farther up the stream in two particulars — it lies at a low angle and 

 is composed of very soft yellow sandstone. It is about 6 feet thick at 

 the thickest part, and at tlie south end of the exposure (the right side of 

 figure 1, pLate 23) it seems to conform with the shale beds above and 

 below it. Toward the north end of the exposure it cuts obliquely across 

 the shale beds ; it forks at one place and has shale between the two 

 branches, as may be seen at the left side of the figure. 



At 19, 75 feet downstream from 18, is a 2*-foot intrusion of soft sand, 

 standing vertically and exposed in the bank at the east side of the creek- 

 This intrusion is exposed for 4 or 5 feet above the waterlevel, and at its 

 top contains man}^ shale inclusions. Immediatel}^ above it the shale is 

 much jointed and broken up, but it is not cut through by the sandstone. 

 The shale beds dip away from both sides of the intrusion as though they 

 had been forced up b}^ the injection of the sandstone under high pressure 

 from below, resembling 12 in this particular. These features of the 

 structure can be seen in figure 2, plate 23. 



At 20, which is 25 feet below 19, is a small soft sandstone intinision, the 

 top of which is exposed at the water's edge. No dikes were observed on 

 Graves creek below this point. 



SUMMARY OF TEE GRAVES CREEK DIKES 



Following is a summary of the principal features of the sandstone in- 

 trusions along Graves creek, and the conclusions regarding them : 



1. The dikes occur near the axis of a low S3^nclinal fold, where former 

 conditions were probabh^ favorable to great hydrostatic pressure. 



2. With one exception, the dikes stand at angles varying from 60 to 90 

 degrees. In strike they vary from north 65 degrees east to south 85 

 degrees east ; the strike of the shale varies from north 10 degrees west to 

 north 55 degrees west. The angles formed by the strike of the dikes and 

 that of the shales, where the latter were measured, var}' from 50 to 90 

 degrees. 



3. With three exceptions, the dikes are of liard gray sandstone. Some 

 of them are intricate!}" and irregularlv jointed, and the joints often 

 contain calcite veinlets. The cementing material in the hard dikes is 

 calcite. 



