124 BULLETIN OF THE 
proaching the upper surface as common to all amygdaloids studied 
by him in connection with the copper-bearing rocks of Lake Superior ; 
and these sheets are well known to be extrusive.’. It is to be noted 
here that coarseness of texture, even at the base of lava flows, presum- 
ably depends, other factors being the same, on whether extrusion takes 
place on land surfaces or under water; so that we should expect the 
trap to be much finer in grain when extrusion takes place under water, 
since texture is a function of rate of cooling. 
When a section across the upper contact is examined under the 
microscope, the lamination of the sandstone, which occupies the in- 
equalities in the upper surface of the trap, is seen to conform to the 
general contour of the hollows. This conformity is usually visible in 
the thin section, even when not noticeable in the hand specimen; it is 
of common occurrence in other localities along the eastern ranges, and 
is highly significant of the deposit of the sandstone subsequent to the 
eruption of the trap. Sections of the trap at the upper surface of the 
sheet also exhibit vesicles, more or less open upwards, which are partly 
or wholly filled with stratified clastic deposits, connected with the over- 
lying sediments by narrow necks. In some cases the sand-filled cavi- 
ties are apparently isolated in the trap, but this appearance is owing to 
the fact that the thin section is transverse to the opening along which the 
sand grains filtered into the cavities. The clastic grains occupying 
the vesicles are usually of the most enduring minerals derived from the 
ancient crystalline rocks, on the side of the Triassic estuary : these are 
quartz, various feldspars including microcline, hornblende, and musco- 
vite, cemented together by granular calcite stained red by ferric oxide. 
Small fragments of vesicular trap occur here also, not the least interest- 
ing of the constituents. The grains first deposited are generally ar- 
ranged with their longer axes roughly parallel to the contour of the 
lower portion of the vesicle; grains later deposited appear approxi- 
mately parallel not only to one another, but to the general stratification 
of the main mass of overlying sandstone, and also to the stratification 
in a number of similar vesicles in the upper portion of the trap sheet 
at this point. So highly specialized an occurrence of clastic mate- 
rial in vesicles at the surface of a trap sheet can‘ have but one inter- 
pretation: the trap sheet is extrusive. Like the conformity of the 
sandstone or shale to the upper surface of the trap, the clastic filling of 
the surface vesicles is very characteristic of the eastern ranges, and is 
1 Metasomatic Development of the Copper-Bearing Rocks of Lake Superior, 
Proc. Amer. Acad., XIII., 1877-78, pp. 282, 283. 
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