and Rock Densities at High Temperatures. 37 



quarrying operations of the past several years have been fol- 

 lowing this slab back through the diabase. 



Lewis' explanation of these occurrences is found in the fol- 

 lowing quotation :* " The first step in such a process is seen 

 in Plate XXII, fig. 1, where a thin sheet of the diabase has 

 followed a bedding-plane about a foot below the base of the 

 main sill. If any portion of the intervening sedimentary bed 

 had broken or parted along a joint-plane and the edge had 

 tilted up somewhat against the now of the intruding magma, 

 it would have been raised by the current to a more steeply 

 inclined or even vertical position." But this can hardly be a 

 complete explanation, as no force is suggested which could 

 start the slab tilting up against the flow. The intruding dia- 

 base can not be compared to a stream flowing freely over a 

 flat, irregular bed, and lifting up blocks by its own momen- 

 tum. On the contrary, we must assume that it flowed rela- 

 tively slowly, under considerable pressure from overlying 

 rock strata. Under these circumstances a thin sheet of liquid 

 diabase, branching off from the main mass and following a 

 bedding plane, would indeed exert a lifting pressure on the 

 under surface of the slab of sandstone ; but this pressure would 

 exceed that on the upper surface by only a small amount, 

 namely, an amount depending on the difference in density 

 between the sandstone and the diabase. In other words, the 

 sandstone would be lifted by the same force that lifts a piece 

 of wood which is immersed in water. 



It is possible that the moving current of diabase might carry 

 the slab along after flotation had lifted it from the floor. But 

 this assumption is not necessary in order to explain the fre- 

 quent vertical position of the slabs, since this is the position 

 which any flat object tends to assume when floating up through 

 a liquid. 



Temperature of intrusion of the diabase. — A comparison of 

 the fusing temperatures of the arkose and the diabase yields 

 the interesting fact that the arkose fuses at a lower tempera- 

 ture than the diabase. Nevertheless, in the inclusions actually 

 found there is no sign of fusion. This discrepancy led us to 

 make a more thorough investigation, with the assistance of Dr. 

 Merwin, of the fusion temperatures of the two rocks. This 

 was done by means of quenching tests and microscopic exami- 

 nation, of which the results have already been published else- 

 where, f Briefly summarized, the conclusions were as follows : 



(1) The "basaltic" fades of the Palisade diabase begins 

 to fuse at about 1150°, and enough of it is fused at 1225° to 

 permit the rock to flow readily. (2) The arkose now found in 



*Loc. cit, p, 134,1907. 



|E. B. Sosman and H. E. Merwin, J. Wash. Acad. Sci., iii, 389-395, 1913. 



