76 B. K. EMERSON — DIABASE PITCHSTONE AND MUD ENCLOSURES. 



threads and bands of the glass passed across to unite with the cord of 

 glass which surrounded the adjacent sand grains and which was not 

 in the least affected. The brown non-polarizing glass was also not much 

 affected by this treatment. Strong hydrofluoric acid quickly dissolved 

 all that remained of polarizing minerals, while the fresh brown glass 

 was dissolved more slowly. This cord is thus not calcite or chalcedony, 

 and it seems to be a peculiar form of the glass, perhaps more acid 

 than the rest, caused by its sudden cooling in contact with the foreign 

 grains. This capillary soaking of the glass out into the sand, thus form- 

 ing what seems to be a quartzite with abundant silicious cement, is very 

 abundant in the lower part of the bed. The sand thus cemented by the 

 glass contains sometimes sharp fragments and tears of a glass of earlier 

 formation. It is at times very muscovitic and has a beautiful flow struc- 

 ture, showing that it was a mobile mud when mixed with the glass frag- 

 ments. 



ME BID EN MUD VOLCANO. 



The mass filling the throat of the opening in the trap ridge consists of 

 rounded bomb-like blocks of trap and abraded blocks of various forms 

 of sandstone, often containing glass, with the interstices filled with finer 

 fragments of the same and the whole cemented by glass often devitrified 

 or calcified and by the aqueous albite-diopside-calcite mixture. The 

 hematite and the segerine-augite of Greenfield are wanting and the glass 

 is less abundant than at the southern bed. The glass often sinks to a 

 capillary film between the grains. Sometimes six or seven varieties of 

 diabase can be seen in a single slide (see plate 9, figure 5). 



Primary grains of plagioclase derived from the sands and having the 

 banding characteristic of granitic plagioclase and the large size, angularity 

 and rust covering of the sand grains are extended by a broad layer of 

 secondary plagioclase which projects out into the glass on all sides with 

 irregular tongues, which are untwinned, but which extinguish with the 

 nucleus. In some cases the central grain could be seen to have a posi- 

 tive optical figure in position like that of albite, together with the small 

 angle of extinction of that mineral. 



Among the interesting inclusions in the glass are — 



1. Minute perfectly rounded grains of the finer sandstone found below 

 the trap. 



2. Rounded tears of the red-brown glass. 



3. Fragments of the basal trap, which are densely black from the abun- 

 dance of ore and which are melted on the surface and surrounded by a 

 smooth rim of a lighter brown blebby glass, from which threads go out 

 into the enclosing mass (see plate 9, figure 3). 



