20 IOWA ACADEMY OF SCIENCES. 



sandstone. 'Ihe cycad beds are therefore Cretaceous and belong to Meek 

 and Haydeu's Cretaceous No. 1. 



A considerable thickness of the sandstone at the top of all the higher buttes 

 of the region has been converted into a very hard, brittle quartzite. The 

 process of vitrification has in some instances almost completely obliterated the 

 original structure; in other cases the original grains are seen imbedded in a 

 a secondary deposit of silica. Contrary to the opinion of some observers, I 

 believe the vitrification to be due to conditions that existed in the sea at 

 the time the beds were deposited. The waters were charged with an 

 unusal amount of soluble silica, which was not only precipitated among 

 the,sand grains, converting the whole mass into a homogeneous quartzite, 

 but some of it was substituted for the molecules of wood and other tissues 

 in the stems of cycads and deciduous trees, that by accident were floated in 

 from adjacent lands. The silicified trunks of ordinary trees now found on 

 the lower slopes occupied by the sandstone are very much broken and 

 weathered and polished by long exposure. On the shoulder of one of the 

 buttes a mile or two west of the main cycad field, not far below the level 

 of the vitrified bed, there was noted a silicified log two feet in diameter at 

 the base, twelve feet of the basal part unbroken, with a train of fragments 

 of varying dimensions extending from the smaller end far enough to indi- 

 cate an original length of seventy or eighty feet The fresh appearance of- 

 this specimen, with its fractures all sharp angled and its parts of consid 

 arable length all in their natural relative positions, was in striking contrast 

 with the short, polished, worn, disassociated fragments found in the residual 

 soil on surfaces two or three hundred feet lower. The differences in condition 

 and appearance indicate enormous differences in the length of time the speci- 

 mens have been exposed. The effects on the better preserved specimen, of 

 rain and frost and wind driven sands, with frequent falls from undermining 

 cliffs, during the years necessary to reduce the hill on which it lies to the 

 level now occupied by the fragments with which it is compared, will not be 

 left to conjecture so long as the worn and dismembered fragments lying at 

 lower levels remain to furnish objective illustrations of what those effects 

 have been in the past. These are reasons for the conclusion that all the 

 silicified trunks, including those of Bennettites, came from the same hor- 

 izon, and that that horizon was the vitrified beds near the summit of the 

 Dakota sandstone. 



East of the valley followed in this vicinity by the B. & M. railway, rises 

 Arnold's peak, a high butte, the summit eight hundred feet above the valley, 

 and like the other high points of the region, capped with vitrified sand- 

 stone. The geological structure at the base is concealed, but a mile or two 

 farther north, almost directly east of Minnekahta, the high ridge of which 

 Arnold's peak is simply the most prominent part, reveals at its base the 

 belemnite-bearing beds of the Jui'assic. The plain on which Minnekahta 

 stands is some scores of feet below the top of the Jurassic, and not less than 

 six hundi-ed feet below the vitrified sandstone near the summit of the Dakota 

 group. On this plain a few specimens of Bennettites have been found, but 

 in most cases they were so far decomposed as to fall to pieces when attempts 

 were made to remove them. Again we find some relation between the 

 abrasion and decomposition that the fossils have undergone and the vertical 

 distance they lie beneath the level of the vitrified beds. Assuming that all 



