IOWA ACADEMY OF SCIE^XT.S. 19 



tlie slratigraphical position of liis species uudecided. To settle, if possil)le, 

 the question detiuitely. the writer recently made a visit to the locality that 

 furnished the types of Macbride's species. 



Specimens of Bonnettites are not very numerous in the Black Hills of 

 South Dakota. At all events not very many have yet been brought to light 

 All the individuals at present known have been found in a rather limited 

 area around Minnekahta, a small station on the Deadwood branch of the 

 B & M. railway. By far the greater number, some forty or fifty altogether, 

 were discovered on an area of only a few acres, about four or live miles 

 southwest of Minnekahta. They all lay partly imbedded in the soil on the 

 southern slope of one of the low, rounded, grassy hills that characterize 

 the marginal portion of the Black-Hills uplift. Separating the cycad hill 

 from the next on the south is a comparatively shallow, but steep sided 

 cailoa, supporting a moderately dense growth of Pinus- ponderosa Doug- 

 lass. The walls of the canon reveal the edges of gently folded and tilted 

 beds of sandstone. Sandstones— yellow, brown or red, sometimes in mass- 

 ive, and sometimes in thinner layers— often project above the grassy surface 

 on the gentler slopes above the canon walls; while here and there are high 

 buttes rising two or three liundred feet above the general level, and com- 

 posed of conformable beds of sandstone throughout their entire elevation. 

 A single sandstone formation therefore, extends from the bottom of the 

 small secondary canyons of the region to the top of the buttes; and, though 

 no cycads were seen in place, there is no reason to doubt that it was in this 

 sandstone, at some level, that they wei'e originally imbedded. The sand, 

 stone exhibits the characteristics of the Dakota group of the Black Hills as 

 described by Hayden, Winchell and Newton; still it was thought best not to 

 decide the question of its age on lithological grounds alone. Diligent search 

 during the time at our disposal failed to disclose the remains of recognizable 

 plants or animals belonging to the sandstone in place. Fragments of 

 silicified trunks, probably of deciduous trees, lay loose on the surface. 

 Some of these were mingled with the cycad trunks, and, since the condition 

 of mineralization was the same in both, it was inferred that the silicified 

 trunks of both types had been imbedded under the same conditions, and 

 that they probably came from the same horizon. A short distance east of 

 the cycad field a gray shale, supposed to be the Jurassic of the geologists 

 who have written on the Black Hills, was revealed by an upward arching 

 fold in the bottom of the canyon, but as it contained no fossils judgment 

 was for a time reserved. Three or four miles west of the main group of 

 cycads the ash colored shales, recognized beyond a doubt by Belenuiitcs 

 densus, M. and H., and other characteristic fossils as the Black Hills "Juras- 

 sic" are exposed in full force in the east side of Big Horn basin. The 

 whole thickness of the Jurassic, two hundred feet or more, is thus revealed; 

 while beneath the Jui'assic shales, at the bottom of the basin-like valley, 

 there is an exposure of lied Beds having a thickness of twenty or thirty 

 feet. The rim of Big Horn basin, on the east side at least, exhibits ten or 

 twelve feet of heavy, cross bedded sandstone resting directly on the Jurassic 

 shales. These cross bedded layers constitute the base of the great sandstone 

 formation, to which reference has already been made. The formation 

 extends from the Jurassic shales to the summits of the adjacent buttes. On 

 stratigraphical evidence we are now prepared to recognize it as the Dakota 



