JANUABY 15, 1897.] 



SCIENCE. 



83 



havior of different forms of silica under 

 solvents, some being easilj' attacked, others 

 very resistant. Prof. Shaler raised the point 

 that it was a reaction taking place on or near 

 the surface and not in depth. Prof. Emerson 

 described his observations on the schists 

 and traps of the Connecticut Valley, where 

 quartz and silicates on the surface were 

 resistant, but where exposed in crevices 

 to the drip of mosses and lichens they were 

 deeply affected. Prof. Stevenson cited the 

 sandstones of central Pennsylvania, where 

 some were greatly corroded on the outcrops, 

 whereas others, such as the White Medina, 

 were very smooth. I. C. White corrobo- 

 rated the last speaker, and cited the change 

 of flint to chalcedonj' in the lower Car- 

 boniferous limestones. M. R. Campbell de- 

 scribed the smooth character of the Medina 

 in Virginia at the surface, but its pitting 

 under cliffs where exposed to dripping 

 water. As the following paper was on the 

 same general subject, practically the same 

 discussion continued in connection with it. 



Erosion at Baselevel. Maeius R. Campbell, 



Washington, D. C. 



Many local baselevel plains in the Ap- 

 palachian coal field show a sharp line 

 of demarcation between the level floor 

 of the basin and the surrounding slopes. 

 Since the streams are too sluggish to me- 

 chanically transport the waste of the land, 

 this condition can be explained only by 

 supposing that most, if not all, of the ma- 

 terial which is swept in from the surround- 

 ing slopes is removed by solution when it 

 reaches the floor of the basin. 



Etched quartz pebbles and geodes are 

 evidence that under favorable conditions 

 silica may be dissolved ; therefore it remains 

 to determine whether such conditions are 

 liable to be present on a baselevel plain. 

 The solution of the quartz appears to take 

 place only in the presence of decaying vege- 

 tation, consequently the swampy character 



of such a plain would offer almost ideal 

 conditions for the removal of the silica as 

 fast as it is washed in from the surrounding 

 slopes. The alumina is still unaccounted 

 for, but the question was raised that some 

 similar reaction may take place, which re- 

 moves this compound also. 



Mr. Turner, in discussion, emphasized the 

 preeminent insolubility of the quartz that 

 forms the veins of the Sierras, as shown by 

 its occurrence in the prevailing boulders of 

 the local conglomerates. He stated that the 

 pitting of chalcedony may be due to the 

 solution and removal of limestone. Mr. 

 Keith also supported the insolubility of the 

 quartz and cited the baselevel superficial 

 soils and gravels near Washington. The 

 residual pebbles are derived from the quartz 

 veins, whereas the other minerals of the 

 gneisses are only represented by a red clay. 

 Mr. Hayes, in support of Mr. Campbell's 

 position, mentioned the ponds in the plateau 

 of the Coal Measures of the South, which 

 plateau is deeply dissected by streams. He 

 stated that much silica might be removed 

 and yet no pitting result to indicate it. Dr. 

 Wadsworth mentioned his own observations 

 on the Potsdam and St. Peter's sandstones 

 of Wisconsin, made many years ago. An 

 outer shell of hardened rock forms by re- 

 deposition of silica, and may even be ap- 

 preciable within a year's time. He also 

 mentioned the well recognized removal of 

 silica in the formation of the soft hematites 

 of Lake Superior, and emphasized its ready 

 and great solubility under favorable condi- 

 tions. Professor LeConte laid stress on the 

 solubility of different forms of silica, and 

 Mr. Hayes, in closing the discussion, ad- 

 mitted freely the frequent insolubility of 

 quartz, but still urged the significance of the 

 observations of Mr. Campbell and himself, 

 which, indeed, were unassailable. 



The Origin of Certain Topographic Forms. 

 Marius E. Campbell, Washington, D. C. 



