FEATURES OF TRAP EXTRUSIONS IN NEW JERSEY 309 



and during the long periods of drought the streams dwindled away 

 or sank into the sands. At times, however, torrential storms or cloud- 

 bursts occurred and the sudden rush of waters carried heavy loads 

 of waste material before them. In the dry periods strong winds 

 played an important part. They gathered up every exposed grain 

 of moderate size from the higher ground, and swept it into the valley 

 and spread it out in accumulations which reached great thickness. 

 The floor of the valley was almost fiat, or at most had a gentle 

 slope away from the mountains toward the axis. In the lower-lying 

 portions lay one or more shallow lakes, which in periods of compara- 

 tively large rainfall spread over wide expanses, and in dry periods 

 contracted within narrow limits. 



THE LAVA FLOW 



Over such a region as we have pictured a great lava flow was 

 suddenly poured forth. The points at which extrusion took place 

 are uncertain. There seem to have been no preliminary phenomena 

 such as often precede lava flows. No deposits of tuff or other ejecta- 

 menta, indicative of explosive violence, are found in this region. The 

 lava seems to have quietly welled forth and, as we shall see later, 

 spread out in a thick sheet in a flow which was practically continuous. 



From whatever point the flow may have entered the region its 

 natural course would be along the trough of the valley. The shallow 

 inequalities which existed in the surface offered no serious obstacle 

 to it; the fluid lava filled them and passed on. Such small bodies 

 of water as were encountered were perhaps quickly turned to vapor 

 and driven off. Nevertheless the lakes have left their record upon 

 the molten rock, and though in places the writing has become dim 

 with the lapse of ages, it has proved so nearly indestructible that it 

 can still be deciphered, and the site of the lake beds, and hence the 

 line of central depression of the valley, can be determined with a fair 

 degree of confidence. If we are not able to do this throughout the 

 area, it is because the field has not yet been thoroughly studied with 

 this object in view, or because the structure of the trap is not suffi- 

 ciently revealed in the surface exposures. 



The process of reasoning is as follows: Where the lava covered, 

 arid soil it was practically unaffected by water. The air and the 



