80 B. K. EMEBSON — DIABASE PITCHSTONE AND MUD ENCLOSURES. 



In the second process the lava bed flowed over the muddy bottom 

 quite rapidly, and the heated mud and water have frothed up into the 

 still liquid mass, causing an intimate blending of sand and lava for a 

 thickness above the base of the bed of from 30 to 75 feet and for a dis- 

 tance, parallel to the advancing front of the sheet, of several miles. 



The sudden introduction of so large a volume of water has caused the 

 mass to cool as a spherulitic glass, with a minute crackling, wdrich gives 

 it a pitchy luster and a large content of water (4.72 per cent.), thus form- 

 ing a basic pitchstone which does not seem to have been described before. 



As a further direct influence of the water on the lava, many abnormal 

 forms of trap were made locally. The liquid mud rose in the liquid 

 lava with many explosions, shattering the abnormal mixtures already 

 solidified, and blending them in still more complex mixtures while the 

 newly solidified glass was still slightly plastic. 



The whole is cemented by the remnant of the glass, or an aqueo-igneous 

 stage follows the igneo-aqueous, and a more distinctly hot-water product, 

 consisting of albite, diopside, hematite, and calcite, and aegerine-augite 

 forms the cement. This glass breccia is proved to be an integral portion 

 of the trap sheet by the fact that there is a heavy basal bed of crystalline 

 trap resting upon the sandstone, and the breccia graduates downward 

 into this bed, as it does also upward into the overlying crystalline trap 

 which forms the major portion of the overflow. Sometimes this basal 

 bed is shattered and its parts carried up into the glass and rounded and 

 filled with superficial steam holes by rem el ting. 



At one point the trap sheet was broken through and the mixture of 

 glass, trap fragments, and sand welled up through the orifice as a mud 

 volcano and graduates upward into the ferruginous sandstone. 



Explanation of Plates. 



Plate 3. — Inclusion of Mud in upper Surface of Trap Sheet. 



Figure 1. — A block of trap from the contact of a sheet of sandstone 12 feet long and a 

 foot wide, which was included in the trap a few feet below and 

 parallel with the surface. The lower surface of the specimen was in 

 contact with the sandstone. The whitest spots are steam holes, 

 filled by secondary calcite. The trap is full of drops and lobate 

 masses of the gray mud. From the north end of the east wall of the 

 cut. Dibbles crossing, Holyoke. About two-thirds natural size. 

 From photograph. See page 61. 



Figure 2. — Polished surfaces of pieces from the south end of the cut to show 

 the intimate mixture of the shattered trap and the light gray mud. 

 The mud fills many of the steam holes in whole or part. Natural 

 size. From photograph. See middle of page 62. 



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