ABSTRACTS 10 1 



height that resemble craters, but that are solid lava. Black Rock 

 Butte has no cone. The lava sheet is more or less dissected, but the 

 fragments are found but a very short distance from the escarpment. 

 Specimens from all the buttes, and from various parts of the southern 

 one, show that while the leucite is almost always present it varies 

 greatly in amount and may be replaced almost entirely by sanidine. 

 Haiiyne was observed, and in Black Rock Butte large augites with 

 surrounding borders of the peculiar yellow biotite. All these points 

 of petrographical interest were illustrated by lantern slides from 

 photo-micrographs. The two published analyses were cited, and in 

 view of the abundant sanidine the rock was called a leucite-phonolite 

 rather than a leucitite. The cones were explained as due to the 

 upwelling of a viscous lava in the final phases of activity. No tuffs or 

 dikes were met. Pilot Knob, an isolated butte lying twenty miles 

 further west and north of Rock Springs, was also described, and the 

 rock was shown to consist of small needles and larger crystals of 

 augite in an isotropic groundmass that was regarded as glass. The 

 determinations of the rock as trachyte in the Fortieth Parallel Survey 

 must have been due to a confusion of slides. 



T/ic Pre-Cambriafi Topography of the Eastern Adirondacks. By J. F. 

 Kemp. 



The paper opened with a review of the geological formations in 

 the Lake Champlain Valley. It was shown that Palaeozoic sediments 

 formed all the eastern shore except for a small area at the south. In 

 Vermont the Georgian strata of the Cambrian are met, but Acadian 

 representatives are lacking. The Georgian is only seen at a distance 

 from the crystalline nucleus of the Adirondack Archrean (Algonkian) 

 Island, whereas the Potsdam and Ordovician lie now well up on its 

 flanks. Prolonged search has failed to show on the west shore of Lake 

 Champlain anything below the Potsdam. Mr. Walcott has already 

 pointed out these relations. The writer stated that the crystallines 

 had all been formed and greatly metamorphosed long before the Cam- 

 brian times, whose strata on their flanks lie flat and show no meta- 

 morphism whatever. He further stated that the crystallines must have 

 been land with an encroaching shore line during the Cambrian, and 

 that the land was deeply carved by erosion, whose results he freely 

 admitted had been greatly modified by faulting. Then, with a series of 

 lantern slides based on the topographic maps of the United States 



