110 B. K. EMERSON — PLUMOSE DIABASE AND PALAGONITE 



Again, it is perhaps conceivable that the diabase just become solid 

 may have been cracked by the many explosions, and the adjacent holy- 

 okeite, still liquid and thus at a much higher temperature than the 

 barely solid diabase, may have filled the fissure and have imparted heat 

 enough to the walls to cause a refusion and coarser crystallization adja- 

 cent to itself. 



A further difficulty arises from the fact that while the slightly coarser 

 diabase on one side of the holyokeite (e) grades outwardly into the 

 normal diabase (a), on the other the coarsest plumose texture is con- 

 tinued for 3 inches and abuts against the black diabase (c) without 

 transition, the great needles rising from the surface of the black diabase 

 as from the surface of the holyokeite. 



The surface of contact on the black diabase (c) is a plane from which 

 the great crystals spring, making the impression strongly that it was a 

 solid surface when they were implanted on it. On the other hand, on 

 contact with the holyokeite (e) the blades spring from the surface of the 

 latter commonly, but sometimes penetrate the holyokeite as if the two 

 had been plastic together. 



The thin band of black diabase seems to be excessively rich in iron 

 and to be the correlative differentiate of the white acid holyokeite, and 

 how this comes to occupy its present position and to be separated from 

 the country rock by a thin vein of quartz, which sends branches into 

 the black rock, is hard to see. 



The preceding partial and tentative explanations may be taken rather 

 as expositions tending to make clear the great complexity of the curious 

 dike. The hypothesis that best correlates the facts would be as follows : 

 The newly solidified trap (a) was fissured and the black extra basic 

 trap (6), made basic by a slight differentiation in an immediately adja- 

 cent portion of the magma, was injected and solidified. A later explo- 

 sion reopened the fissure along its left-hand wall three inches wide and 

 the magma filled it. The surface of the highly pyroxenic diabase (c) 

 stimulated the growth of the pyroxenes in this magma, and they shot 

 out nearly across the cavity, forming the long-plumose diabase (d) and 

 thrusting, as it were, the more acid portion of the magma across to the 

 opposite wall, where it solidified as the holyokeite. The definiteness of 

 the plane between c and d agrees well with this, and the good degree of 

 definiteness of the other boundary with exceptional blades of pyroxene 

 penetrating the holyokeite (e) would also comport well with this de- 

 scription. The slight increase of texture of a, immediately adjacent to e, 

 may be due to the reheating from the introduction of the new magma. 



The quartz (6) is an abundant last intrusion everywhere, and a last 



