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 gi-ain 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 iu 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 uj)per 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 tliis 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 mnss 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 occiu'rence of chistic mate- 

 rial in vesicles at the surface of a trap sheet can have but one inter- 

 pretation : the trap sheet is extrusive. Like tlie conformity of the 

 sandstone or shale to tlie 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., XIIL, 1877-78, pp. 282, 283. 



