THE DEEKFIELD SHEET. 423 



(if the waters of the mud on tlie heated lava under pressure. It appears as 

 narrow liuij)id l)ands in A and 7>, often interjected between the otlier con- 

 stituents. It is made up mostly of a clear feldspar, in blades and ])lates sev- 

 eral times twinned, of very fresh appearance, and polarizing in bluish whites, 

 with the strono-, wavy or central extinction which characterizes the water- 

 deposited albites of the cavities of the red diopside-diabase of the dike at 

 Cheapside (fig-. 24, 6'). This feldspar is also closely like the ordinary pla- 

 gioclase of the amphibolites and albitic schists of the metainorphic series 

 farther west. There is also a pyroxenic mineral of a quite peculiar character 

 associated with this feldspar, and like it plainly of secondarv origin. It has 

 extinction a = emerald-green, b = clove-brown to violet, c = red-brown. 



In this rock small groups of stout, colorless diopside crystals occur, 

 often bi'istling on the surface of the sand filaments like the hematite (whicli 

 is another constituent <if this rock), and in one case a well-foi'med arrow- 

 headed twin of this mineral was observed. The considerable development 

 of the green pyroxenic mineral gives much of the tuft-like rock a green 

 color and the appearance of being greatly weathered dialjase, and this 

 somewhat abnormal variety forms narrow and interrupted bands l)etween 

 the filaments of the red mud and small fragments of tlie trap. These latter 

 have the primary and secondary feldspars weathered and inclosed in an 

 olive-green groundmass. The hematite plates penetrate to the verj- center 

 of these fragments. 



The mvid was thus most intimately blended with the liquid traj) in 

 which the lath-shaped feldspars had already been crystallized. It furnislied 

 water for the livdi-ation of the groundmass into an olive-green nonijolarizing 

 glass, and some of the same superheated water produced the abnormal 

 igneo-aqueous deposit wliich unites the normal trap with the sand filaments. 



Several vears after the foregoing description was written I made a 

 comparative study of the above occurrence and similar tuff-like beds in 

 Meriden, Connecticut, during which many slides were examined and an 

 analysis of the glass at Meriden was made. This gave me much clearer 

 ideas of the part taken by the water in forming and shattering the glass 

 (which proves to be a basic pitchstone) to make the fine sand and trap- 

 breccia mentioned above, in carrying" up portions of the basal bed to 

 become the bomblike masses, and in promoting the formation of a rock 

 resembling a crystalline schist. I therefore reprint here the substance of 



