PETROGRAPHY OF THE VARIOUS BEDS. 69 



pearance. Under the microscope their tufiaceous character is well shown ; vesicular 

 porphyritic trap grains abound, and others of non-polarizing character are derived 

 from yellowish glass, now wholly or partly devitrified." 



Petrographical Descriptions. 

 surface of beds at dibbles crossing and larabee's quarry. 



At Dibbles crossing the mudstone which has passed into the trap is 

 mottled with brighter spots where the calcite is concentrated in minute 

 concretions, forming a partial oolitic texture. 



The trap shows the two generations of plagioclase in a marked degree 

 in a basis which seems to be devitrified glass, but made black by fine 

 granular black ore. The iron being altogether in this form, augite is 

 wanting. 



Sometimes when the mud has shrunk away from the trap a drusy 

 crust of obtuse scalenohedra of calcite appears under the microscope ; 

 upon this a coating of siderite or ankerite rhombohedra, now rusty in 

 cleavage planes, and above this a crust of quartz prisms. The trap con- 

 tains pores filled with mud with secondary calcite. Other round, j^ale 

 green spheres of green glass polarizing faintly in a small portion of the 

 surface are of problematic origin. 



At Larabee's quarry the black mud is intimately mixed with the trap, 

 which shows large and small feldspars in a beautifully tufted hyalopilitic 

 basis or in a basis of devitrified glass black from magnetite grains. The 

 mud produces a minimum effect upon the trap; only a black, dense 

 surface of the cavities is produced. The mud often lines the whole of a 

 cavity as a thin film (apparently by the expansion of the steam pressing 

 it against the walls). The center is filled by coarse calcite. 



BASE OF HOLYOKE BED. 



At the foot of Titan's pier the same drusy crust as above occurs. There 

 is also a microscopic layer of a curdled impure glass at the surface of 

 the trap fragments. 



Sections cut from the base of the Holyoke bed at the peak west of 

 Norwottock show a greater amount of change. A layer of bright green 

 glass, with sharp, small feldspar rods and black magnetite dust, encloses 

 and penetrates the red sand. This glass contains also distinct crystals 

 of calcite. A narrow band of this glass adjacent to the sand is slightly 

 devitrified in fibers parallel to the sand surface. Next outside this a 

 band of magnetite grains separates it from a band, showing an indis- 

 criminate mixture of the green glass in small, irregular patches, with a 

 transparent granular calcite. With polarized light the glass is seen to 



