9-8 PROCEEDIXOS OF THE AXX ARBOR MEETIXG 



approximately quantitative estimate of the importance of tlie several factors 

 involved is to be made. 



Presented in abstract extemporaneously. 



Discussiox 



I. C. White : I found the same series of lavas in a plateau in Brazil of 

 thousands of square miles in extent and exhibiting great massive cliffs. A 

 road that the Brazilians built up the cliffs to the great coal fields and the 

 cattle country cost $1,000,000. It is simply a mule track cut up the sides of a 

 huge cliff. The dikes from this great outflow penetrated the coal bed, and T 

 traced a finger of dike along the immediate contact with the coal bed for 100 

 feet and found it had very little metamorphic influence on the coal. It had 

 nearly as much volatile matter as original coal, but only touched the general 

 metamorphic region. The lava was not very hot. The dike was only about IS 

 inches thick. The same series is found there as occurs in the Gunawanda? 

 country, going over Cambrian? and the underlying Mesozoic beds. 



H. S. Washingtox : I might add a few words. There are two groups, one 

 containing 48 and 49 per cent of silica, connected by very hard iron, about 14 

 to 1.3 per cent. Stormberg basalt contains about 52 per cent of silica and the 

 other about 53 1/^ per cent. They are decidedly low in iron and the most 

 silicious has only dVo per cent of iron and the other has only 8 per cent. 



Fred. E. Wright : One very clearj- sees that iron has very little to do with 

 it and usually iron is only one factor that enters into the problem. It is char- 

 acteristic of all basalt lavas that they came up with very little explosive 

 action. Explosive action occurs chiefly in hydrous magmas. It seems to me 

 that iron with other basalts is one of the important factors : the other is the 

 presence of gases which escaped on account of the high fluidity of the basalts. 



L. C. Graton : I gather from the reference to a colored top that it was some- 

 what of a different composition than the other composition analyzed. Is that 

 on account of retention of ferric ratio? If the lavas are in the attitude in 

 which they were poured out, was there any slumping? With Dr. Wright's 

 familiarity with Michigan lavas, would he say they are of the plateau type or 

 would their slumping suggest somewhat different mechanism? 



Fred. E. Wright : The color of the small flows is red. many times a very 

 intense surface film of bright red. The ordinary lava flow coloration is not 

 so intense. In regard to the .slumping, there is a slight tendency toward slump- 

 ing of several degrees toward the center of the basin which might be due to 

 the weight. It is nearly horizontal. The point I made yesterday was in con- 

 nection with the Keweenaw flows, where you have flow of uniform thickness 

 and to a great extent almost subaerial and flows off on horizontal basis. It 

 seems to me that Professor Kemp's photographs yesterday were lava flows 

 instead of lava sheets. One was low temperature and stiff, giving out gas. 

 whereas plateau basalts welled out, came to rest, and, being thin, came to rest 

 horizontally. 



