126 BULLETIN OF THE 
the back of the Totoket sheet, locality 16, is similarly indurated, and 
shows as little indication of baking. 
In review, it may be said that the absence of any tendency towards 
a finer crystalline texture 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 vesicles 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 profitably 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 the advancing flow, when its thickness was not so great as 
afterwards. Next may be mentioned the general holocrystalline, non- 
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 scoriaceous texture of the upper portion must have 
favored quick cooling and subsequent alteration in the eastern sheet, 
without normal subaerial weathering; the effect of the presence 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 be 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 be 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, /oc. cit. 
