228 Notice of Messrs. Poster and Whitney^s Report ^ 



, The wp-per portions of the sheets of trap are highly vesicular, 

 resembling pumice. Fragments of amygdaloid, sometimes round- 

 ed, at others angular, are found enclosed in the pumice-like trap, 

 as though they had become detached and afterwards reunited to 

 the mass, while in a molten state. Numerous short and irregular 

 fissures, extending to no great depth, are observed on the upper 

 surface of the trap, in which sandstone has been deposited. * * * 

 Between the sandstone above and the trap helow^ it is extremely 



difficult to determine where the one beains and the other ends. 



Fragments of amygdaloid, angular or partly rounded, are included 

 in the sandstone — more numerous near the base than at the top 

 of the deposits. Where the sandstone is imposed on the trap, 

 there is little evidence of its having been metamorphosed ; but, 

 on the other hand, where the trap rests on the sandstone, the line 

 of junction is clear and well defined. The trap is less vesicular; 

 and the upper portion of the sandstone belt, for the distance of 

 three or four feet, is converted into a ribbon jasper, having a com- 

 pact texture. These phenomena have been observed at numer- 

 ous places both on Isle Royale and Keweenaw Point. The beds 

 of sandstone are not shattered, nor does the igneous rock pene- 

 trate in the form of dikes or ramifying veins. All the phenom- 

 ena indicate that the igneous rocks were not protruded in the 

 form of dikes between the strata, but that they flowed like lava 

 sheets over the pre-existing surface; and that the sand was de- 

 posited in the fissures and depressions of the igneous belt, in 

 some cases, while the mass was in an incandescent state.'* 



Similar alternations occur in the cliffs of the shores, consisting 

 of different igneous rocks. 



" At another point on the coast the following: section was ob- 

 served : 



Feet. Inches. 



Compact trap, breaking into cuboidal blocks, 15 

 Porphyritic trap, - - _ . 



Compact trap, 

 Porphyritic trap, 

 Compact trap, 



Porphyritic trap. 

 Compact trap, 



3 



4 



2 



1 6 

 1 



J. ^ 



23 5 



These alternations exhibited well defined lines of junction, 

 and preserved their parallelism along the face of the cliff, as far 

 as exposed. The bearing and dip were the same as in the sec- 

 tion before described. The lines of division pursued an undevi- 

 ating course through the several bands. 



A small vein was observed at one point cutting vertically 

 through these bands, and the veinstone exhibited marked changes 

 m Its passage through the different belts. 





