126 BULLETIN OF THE 



the back of the Totoket sheet, locality 16, is similarly indurated, and 

 shows as little indication of bakinsf. 



In review, it may be said that tlie absence of any tendency towards 

 a finer crystalline textnre in the trap immediately at the contact with 

 the larger sandstone areas, the highly vesicular texture of the upper 

 portion of the mass, the parallelism of the axes of the sand grains and 

 of the lamination of the deposits that they form, the connection of the 

 sand filled vesigles by narrow necks with the sandstone above, and the 

 occurrence of trap fragments in the overlying sandstone beds, all point 

 to the extrusive origin of the trap sheet, and to the subsequent depo- 

 sition of the sandstone upon it. After reaching the conclusion that 

 the Saltonstall sheet is extrusive, it may be prohtabl}' compared with 

 the West Rock and Gaylord Mountain sheet. The first contrast to be 

 mentioned, and the one most conspicuous in the field, is the presence of 

 numerous vesicles in the upper portion of the eastern sheet, and their 

 absence in the western : this we would refer to the small pressure 

 upon the surface of the extrusion at the time of its cooling. The 

 few vesicles near the base of the sheet may have been produced at 

 the front of tlie advancing flow, when its thickness was not so great as 

 afterwards. Next may be mentioned the general holocrystalline, n(jn- 

 porphyritic, and relatively coarse texture and the small degree of altera- 

 tion of the western sheet, while the eastern is more or less glassy and 

 porphyritic throughout, and greatly altered. The eruption into water 

 and the highly seoriaceous texture of the upper portion must have 

 favored quick cooling and subsequent alteration in the eastern sheet, 

 without noi'mal subaerial weathering ; the effect of the pi'esence of 

 much glassy base must also be considered, for this is peculiarly prone 

 to alteration ; it is now as a rule wholly devitrified to chloritic sub- 

 stances, microliths, ferrite, etc. But none of these factors could affect 

 the intrusive sheets ; their imprisonment between the beds deep be- 

 low the surface would allow them to cool slowly, and thus acquire 

 a coarse texture, and would decrease the rate of hydration and altera- 

 tion ; for these reasons we find them preserving in a great degree their 

 original characteristics. It should he noted, however, that inasmuch as 

 a thin extrusive sheet is vesicular through a greater proportion of its 

 mass than a thick sheet, thick lava flows may bo much less altered 

 than thin ones. Thus the heavy sheet of Mount Tom in Massachusetts 

 is practically anhydrous, while the thinner sheet of Saltonstall Mountain 

 contains 3.9^ of water.-^ Finally, there is a most marked and per- 



1 Hawes, loc. cit. 



