312 



PRINCIPLES OF STRATIGRAPHY 



of the igneous and non-igneous rocks, and not infrequently a trans- 

 formation of the basal part of the flow. Such features have been 

 described from the base of the Newark trap sheets of the Paterson 

 region of New Jersey by Fenner (ii). He finds that in places 

 "along the contact for a width of ten feet or more the trap and 

 mud show evidence of having experienced the most violent agita- 

 tion" — the two being mixed and kneaded together in a surprising 

 manner. "The mud has boiled through and through the seething 



Fig. 53. Bouldery structure, with in- 

 cluded sand masses brought 

 from below. Trap sheet 

 near Paterson, N. J. (After 

 Fenner.) 



Fig. 54. Bouldery structure in 

 lower part of extruded 

 trap sheet, near Pater- 

 son, N. J. (After 

 Fenner.) 



body of lava until particles of mud of every size from minute specks 

 to large masses have become incorporated in the pasty flow. Both 

 lava and mud are full of blowholes, steam vents, and other forms 

 of irregular pipes and cavities which attest the violent escape of 

 gases." (ii:jj?.) The amount of injected mud decreases up- 

 ward and the vesicular character gradually gives way to purer 

 igneous rock. In structure this is still very peculiar, showing the 

 effect of escaping gases through the mass. A boulder-like structure 

 is produced, the "boulders" being of dense trap with crusts of dark 



