DISTRIBUTION AND MODE OF OCCURRENCE 629 



apparently fed by the dike that joins it on the west. Its present shape 

 suggests that it was of small lateral extent and relatively thick, so that it 

 was of laccolithic habit; but since none of its cover is preserved this can 

 not be positively determined. 



There are many dikes and thin sills that cut the sedimentary rocks, 

 some of which seem to connect with the Gettysburg sheet or the cross- 

 cutting bodies; but others cut these rocks and are therefore of slightly 

 later age, probably representing the last outburst of igneous activity 

 from the same source. The dikes follow joint fractures, but no system 

 seems to prevail, although the great number, especially the longer ones, 

 trend nearly north and south. A network of such dikes and sheets occurs 

 3 miles northeast of Gettysburg. A chain of nearly continuous north - 

 south dikes of great length crosses the area. The northern one of these 

 dikes, which leaves the overlap-contact sill west of York Springs, has been 

 traced across South Mountain into the Cumberland Valley and is known 

 to continue across North Mountain— a distance of at least 25 miles. 

 Another dike, whose connection with the former is not established, runs 

 south from Chestnut Hill and joins the Gettysburg sill — about 15 miles 

 distant. A third dike in the chain leaves the south end of the Gettysburg 

 sill and has been traced for 40 miles across Maryland. These three dikes 

 and their intrusive connections have a total length of about 00 miles. 



CONCLUSIONS AND HISTORY OF TBIASSIC EVENTS 



It is believed from a detailed study of the rocks and their relations 

 that the progress of Triassic events was somewhat as follows : The sedi- 

 mentary rocks were deposited in a sinking basin not directly connected 

 with the sea; the sinking began in the southeast and progressed north- 

 westward, so that the center of the basin of sedimentation shifted north- 

 westward with the progressive sinking and the sediments overlapped on 

 the Paleozoic limestone floor in that direction, the beds being tilted uni- 

 formly westward by the progressive sinking. The deepest part of the 

 basin at the close of sedimentation was near the northwestern side, where 

 the latest sinking had occurred, but not at the extreme northwest edge 

 of the basin where the sediments are now seen overlapping nearly hori- 

 zontally on the fiat limestone floor and apparently have not been tilted 

 by sinking The sinking of the basin was accompanied by fracturing and 

 probably faulting of its floor, particularly where the greatest sinking oc- 

 curred. The progressive sinking of the basin and the later great depres- 

 sion of the block by drop faulting along its northwest edge were due to a 

 lack of support from the rocks beneath, caused by internal movements of 

 readjustment of equilibrium. Tin's condition was accompanied by a re- 



