36 H. B. KUMMEL 



sive sheets, is entirely absent. Locally, the shale is slightly 

 altered for a few inches beneath the trap, but even this is not 

 always the case. When this is compared with the intense altera- 

 tion which has affected the shales beneath the Palisades, an 

 intrusive sill, for a distance of over ioo feet, the difference 

 between the sheets is emphasized. 



Upper contacts have not been observed in many cases, but 

 the upper surface of these sheets is frequently vesicular, amyg- 

 daloidal, and scoriaceous. Locally, a thin layer of waterworn 

 trap particles, intermixed with red mud occurs between the 

 vesicular trap and the unaltered typical red shales, or the vesi- 

 cles are filled with the red mud. The overlying shales conform 

 to the slightly irregular, ropy surface of the trap. In frequent 

 exposures the rolling-flow structure, named by the Hawaiian 

 Islanders Pa-hoe-lwe, is visible. Nowhere have any tongues of 

 lava been found extending from the main sheet into the neigh- 

 boring shales. 



In texture there is a marked difference between the overflow 

 sheets and the intrusive sills. Not only is the trap vesicular and 

 even scoriaceous at many points on the upper surface, but it is 

 uniformly of much finer grain than that of the Palisades and 

 similar ridges. Microscopic examination of fragments from 

 the upper surface shows that volcanic glass occurs to some 

 extent. The conclusions drawn from the texture are that 

 these masses cooled much more rapidly than did the Palisade 

 trap. Locally, vesicular and scoriaceous layers occur next to the 

 under-shales, beneath dense, fine-grained trap. Generally in such 

 localities the rolling-flow structure is clearly marked. The infer- 

 ence from these facts is that as the lava flowed onward the par- 

 tially cooled vesicular slag-like and broken upper surface was rolled 

 over to the under side of the flow, or in other cases, after a clinker- 

 strewn crust had formed on the top and front of the sheet, the 

 molten lava broke forth from within and flowed over and around 

 the scoriaceous fragments, and on hardening bound them firmly 

 together. Occasionally the red shale rises into the base of the 

 trap, as if the great pressure and flowing motion of the molten 



