EFFUSIVE IGNEOUS MASSES 313 



glass. (Figs. 53, 54.) Between these "boulders" lie vugs of min- 

 erals especially rich in zeolites. 



Emerson {10:61) has described inclusion of "mud drops" or 

 mud amygdules in the upper part of the Triassic trap of the Hol- 

 yoke region in the Connecticut \ alley, where occur also the gray 

 laminated shales confusedly mingled in the trap, and appearing 

 under the microscope as an intimate mixture or "complete emulsion 

 of the two non-mixing fluids," the lava and the mud. He finds that 

 the base of the sheet has similar pronounced mud inclosures in 

 one place, while at another, a hundred rods north, "along the base 

 of the same sheet the black compact aphanitic trap rests on the 

 same coarse sandstone, and contains only a few long steam holes." 

 Emerson regards the basal layer as the underolled top layer. 



The Carbonic lava flows of the coast of Fife, Scotland, show 

 in their bases a series of pipes or funnels which were evidently 

 formed by the steam generated when the lava flowed over the wet 

 sands. These pipes extend vertically into the lava and generally 

 are filled with zeolites or with calcite. 



The rhyolitic flows of the Yellowstone National Park rest in 

 places upon a deposit of well-laminated rhyolite dust, while the 

 basal part of the flow itself along the contact is marked by a thin 

 layer of perlitic glass. (Iddings-i8 : J55'.) In the Grand Canyon 

 of the Yellowstone the bottom contact of a younger flow of rhyo- 

 lite on a thick mass of basalt shows a tuffaceous character passing 

 upward into denser material which in turn passes up into porphy- 

 ritic glass, and this into lithoidal lava. (18:590.) Another small 

 sheet of rhyolite on Saddle Mountain has its basal contact on basal- 

 tic breccia, marked by white rhyolitic tuff, followed by fissile light 

 gray lithoidal rhyolite with small phenocrysts, passing up into dark- 

 colored spherulitic and glassy rhyolite with lithophysse and small 

 phenocrysts. (i8:jp2.) 



Features of the Upper Surfaces of Lava Flozvs. The upper 

 surfaces of lava streams vary with the nature of the lava itself and 

 the conditions under which it is extravasated. 



I. Basic lavas. These are well illustrated in the character of 

 the extravasations of the Hawaiian Islands. The kinds of sur- 

 faces represented by these lavas may be classified as (a) ropy, 

 (b) the pillozvy or pahoehoe and the rough or aa. 



The ropy lava, the least common, has the aspect of irregular 

 coarse pieces of rope, generally intertwined in an extreme manner, 

 the strands moreover being longitudinally ridged. The rock is 

 moderately vesicular. The pillowy or pahoehoe (pr. pah-hoy-hoy — 

 literally, with satiny surface) has been compared with the pitch 



