58 



part of this mass is a compact and, to the naked eye, structure- 

 less petrosilex, of a greenish color. Chemical analysis, too, 

 shows that the "toad stone" has the composition of a true 

 petrosilex ; a slightly weathered, but otherwise typical, specimen 

 aifording Miss E. M. Walton the following result : — 



SiOa 



77.200 



Al.Oa 



12.482 



re,03 



. . 1.570 



CaO . • . 



.800 



K2O 



1.230 



Na,0 . . ... 



4.423 



H2O 



2.004 



MnO 



trace 



99.709 



As in the banded petrosilex, there is here a partial separation 

 of the quartzose and feldspathic materials. Instead, however, 

 of forming thin sheets or laminae, we have one of the con- 

 stituents — the feldspar — segregated into small spheroidal, 

 ellipsoidal, or almond-shaped masses ; while the interstices 

 between these are filled with the quartz, — not necessarily 

 pure silica, but quartzose material, — wliich is usually small 

 in amount. The feldspathic aggregates sometimes appear of 

 very irregular shapes and sizes ; but usually they are regularly 

 rounded, of uniform size in the same part of the rock, and have 

 their longer axes parallel, giving the mass a faint appearance 

 of stratification. At one point, too, they are arranged in lines, 

 which are straight and parallel ; or, more exactly, the quartzose 

 material is in thin layers, and each attenuated stratum has a 

 layer of feldspathic nodules on either side of it. The nodules, 

 thus arranged, sometimes coalesce, forming continuous laminae, 

 and reproducing the banded structure. The strike and dip, as 

 indicated by the parallel arrangement of the nodules, appears 

 to be the same in all parts of the rock, and agrees with that 

 recorded above for the petrosilex generally. The almond- 

 shaped masses vary in size from mere specks to one-fourth 



