THE POSTEEIOK SHEET. 4(39 



These pass upward in fissures for about a foot and then bend the unbroken 

 layers of the sandstone above into an arch, forinin<>- minute hiccoliths, and 

 clearly indicating that the sill was injected under strong jjressure. 



SILLS INTRUDED IN THE SANDSTONES BELOW THE POSTERIOR SHEET. 



Besides the dikes and sills which penetrate the sandstones so abundantly 

 in the immediate vicinity of the Little Mountain core, other small sills 

 appear immediately beneath the posterioi- sheet at so great a distance that 

 they can not be brought into very close connection with the core itself 



The most northern of these is N. 60° W. of the Smiths Ferry station 

 and about G feet below the top of the sandstone. There is a sill 2 feet wide 

 which can be followed 20 feet, and 2 feet below this is another only 1 foot 

 wide. The sandstone has strike N. 40° E. and dip 22° E. 



About a mile south along the bluff, at a point S. 65° W. of Smiths 

 Ferry and west of the marked drumlin which conceals the tuff, a larger sill 

 appears, 10 feet below the trap, which is 4 feet wide and 8 or 9 feet long-. 

 The sandstone is much disturbed Ijeneath it. 



Along the boundary of the sheet farther south no other sills are found 

 in the sandstone below until the western border of the plug is reached and 

 the very abundant dikes and sills appear around its western and southern 

 side, which have been described and fig-ured" above. 



There is a turnstile by the road, and steps going down to the railroad, 

 a mile and a half below Smiths Ferry, and the field road southwest from 

 here leads out over a ridge to an amphitheater, now called Forest Park, 

 from which all the points here described are easily identified. The ridge 

 is the continuation of the trap sheet southward. Tlie beautiful horizontal 

 12-foot sill described above (p. 468) is in the north wall at one's right, and if 

 one crosses the basin to the next ridge overlooking the reservoir, and west of 

 the terminus of the Electric Road, the high bluff of the plug projects south 

 toward the point where one stands, and above the screes of trap fragments 

 the sandstone veneering can be seen abutting against the trap in the thick 

 woods. 



Turning south from the east end of the 12-foot sill mentioned above, 

 on the southeast of the plug, and going to the bottom of the basin near 

 the brook, one finds a place where the sandstone is crushed into sharp folds 

 a foot or two across and baked by the trap, which has penetrated it irregu- 

 larly, but apparently only in small amount. 



