378 R 0. Hovey—Trap Ridges of the 



which form the range next west of Pond Rock show as much 

 hydration as any part of the great ridge itself, so far as 

 macroscopical examination goes. The lavas of Kilauea show 

 no hydration of their constituents, and other modern lavas are 

 like them in this respect. Daubree has shown experimentally 

 that liquid rock under great pressure will absorb into its 

 mass vapors from surrounding material, and hence that 

 the vapors formed by liquid rock ascending through wet 

 porous rocks may find readier means of egress with the moving 

 mass than back through the interstices of the porous rock 

 against hydrostatic pressure. The present hyclrated or non- 

 hydrated character of the trap, therefore, is not dependent 

 upon its being an overflow sheet : it may be due to the nature 

 of the strata through which it passed. 



Better than all this negative evidence as to the character of 

 Pond Rock is the positive testimony of the overlying sand- 

 stone and shale. Wherever these are exposed near or against 

 the trap their dip is much higher than that of the rock 

 underneath the sheet (vid. pp. 28, 29 supra) ; the difference of 

 dip between the underlying and the overlying strata in the 

 railroad cut being 35°, and on the northern hook of the ridge 

 17°. The overlying sandstone on the northern hook is hard 

 baked, and at the contact is mixed with the trap in such a way 

 as to show that it must have been laid down before the advent 

 of the trap (vid slide No. 18), and also that the difference of 

 dip cannot be ascribed entirely to lateral shoving since the 

 eruption of the trap. These facts appear to sustain fully the 

 conclusion that Pond Rock is an intrusive sheet. 



The breaks in the southern part of the ridge, near the rail- 

 road cut and at the foot of the lake, seem to be due to irregu- 

 larities in the original fissure rather than to faulting after the 

 trap was in place.* The thickness of the trap sheet diminishes 

 greatly and rapidly at each place, leaving a narrow isthmus to 

 connect the larger portions. The sheet thins from 225 feet, a 

 short distance north of the railroad, to 84 feet in the cut and 

 thickens again to 250 feet or more in the second part of the 

 ridge ; while at the foot of the lake the trap is apparently less 

 than 40 feet in thickness. The isthmus at the cut is very 

 short, but from the end of the lake the narrow portion extends 

 more than 200 feet southward. If these were faults we 

 should expect to find approximately the same thickness of the 

 trap sheet on both sides of the fault-plane, unless the trap on 

 one side had suffered more from erosion than the other. That 

 the narrowing in Pond Rock is due to the original fissure and 

 not to erosion, glacial or other, is proven by the presence of 



*Prof. Davis says that these notches are " oblique faults " in the trap. Tid 

 this Journal, III, xixii, 346, 1882. 



