78 BULLETIN OF THE 



below upwards : " conglomerate rock," " mixed conglomerate and sand 

 rock," and "red sandstone and shales." He regards the first as a " trap- 

 tuff," and as made up of " rounded masses of greenstone and amygda- 

 loidal trap, of which the former make up by far the larger proportion, and 

 scarcely a pebble of any other rock than trap, enters into its composition." 

 The second is composed of the same materials as the first, and is conform- 

 able with it. The only difference is that part of it is made up of sand 

 composed of finely comminuted greenstone. The last, or " the red sand- 

 stone and shales," he considers to differ widely from the preceding rocks, 

 and to be made up of detritus of the granitic and metamorphic rocks, 

 containing, however, some sand that appears to be comminuted trap. 

 This red sandstone extended, according to Houghton, as far east as 

 Grand Island, where it was unconformably overlaid by the " Gray Sand 

 Rock " (e. g. the material of the Pictured Rocks), which rested upon the 

 uplifted edges of the former. It will thus be seen that he had changed 

 his views since his fii'st report. 



All these rocks were said to be traversed by dikes injected parallel to 

 the bedding, varying in width from fifty to four or five hundred feet. 

 He considered that the sedimentary rocks were all deposited prior to 

 the injection of the traps, which rocks he finds very abundant in the 

 conglomerates, and comparatively rare in the red sandstones. These 

 dikes pass from a compact greenstone on the southeast side to an amyg- 

 daloid on the northwest. He considers that they were " in an intense 

 state of ignition while in contact with the sedimentary rocks, as is 

 clearly shown by the very great changes that have taken place in the 

 rocks last alluded to. In fact, I am disposed to refer the origin of much 

 of the amygdaloid rock to the fusion of the lower portions of the sedi- 

 mentary rocks referred to, for the reason, that as we pass south from 

 this junction, the amygdaloid rocks wholly disappear, their place being 

 supplied by greenstone ; and again so intimately are they blended, that 

 it is frequently impossible to determine where the amygdaloid ceases 

 and the upper sedimentary rocks commence. Fragments of the sedi- 

 mentary rocks, the characters of which can be clearly recognized, are not 

 of rare occurrence, imbedded in the amygdaloid rock, a circumstance 

 which although by no means conclusive, should not be overlooked in 

 considering this subject. I would not wish to convey the idea that the 

 amygdaloid rocks have their origin exclusively from the altered sedimen- 

 tary rocks, but simply that the change in the structure of the trap, from 

 greenstone to amygdaloid, may and no doubt does depend upon the 

 proximity of the sedimentary rocks to the trap, while the latter was in 



