THE DEEKFIELD SHEET. 421 



about 7 feet above the saudstoue aloug which the veins of red sand blend with 

 a fine-grained, reddish material quite unlike the coarse blocks of trap, and 

 this reddish material cements the blocks of earlier trap together for a few feet 

 higher, and higher up the blocks grow more distant and smaller and disappear 

 in the naass of the newer material, which is cracked into small fragments, so 

 that the whole closely resemldes a tuff, but is not a tutf, if the idea of trans- 

 portation of fragmental igneous material by air or water be essential to the 

 definition of a tuff". It is a breccia of sand, trap fragments, and glass, pro- 

 duced by explosions of the water introduced with the nuid. In places it 

 loses the red color and becomes greenish. On examining tlie whole face of 

 the cliff, it is seen that this tuff-like condition continues up half the height of 

 the bed, and its upper boundary continues north and south foi' a long dis- 

 tance. This is visible in the jilate. 



A careful examination of tlie zone of contact of the sandstone veins 

 and the newer trap shows the latter to be compact or finely porous, as 

 contrasted with the blocks of trap, which are very coarse araygdaloidal. 



The newer trap or glass-breccia is reddish, because it is an intimate 

 mixture of trap and red sand, and for 20 feet up, as far as one can climb 

 at the quarry, the mixture of the filaments of sand and trap are most inti- 

 mate, and on a polished surface it is seen that the delicate anastomo.sing 

 films of the trap penetrating the sand could have reached their present posi- 

 tion and condition only in a liquid state, while the thin layers of sand are 

 as intimately mixed in the trap. 



Under the microscope (see fig. 24, A, B, p. 422) the thicker portions of 

 the sand filaments (left side of figures) in specimens taken about 20 feet 

 from the base of the sheet are of the same texture exactly as in the broad 

 intruded masses of sandstone below, but are blackened around their border 

 by the caustic action of the adjacent lava, and as they grow thinner they 

 become black across their entire width. This seems to be caused by the 

 coating of the sand grains with hematite derived from the iron of the red 

 mud and recrystallized by the heated waters. These borders bristle out- 

 wardly also with beautiful hexagonal plates of blood-red hematite, and the 

 same plates are found also in the sand and in the suiTOunding rock. 



The second constituent of the rock is the trap, here in somewhat 

 abnormal development. It is in small fragments and minute filaments, 

 penetrating the sand in every way. It contains the large, angular, and 



