EXPLANATION OF PLATES 127 



The drifting of the long crystals into an adjacent portion of the magma, 

 where they were remelted, would harmonize with Mr Lane's suggestion 

 of a slower crystallization in the schlieren, as would, perhaps, also the 

 general development of an incipient differentiation. 



Explanation of Plates 



Plate 24. — Map of Trap Sheet across Holyoke and Section, 



Showing Holyoke diabase, plumose diabase, palagonite, and surface layer of mud 

 inclosures. The section is along the line AB (see page 95). Scale 

 1 mile to the inch. 



Plate 25. — Tubular Cavities and Block of Trap 



Figure 1.— Tubular cavities (X £). 



a. Tubular cavities formed in the superficial portion of the trap 

 sheet by the escape of steam, b. The same found inverted at 

 the base of the sheet. They illustrate the underrolling of the 

 surface layer of the advancing sheet (see page 94). 



Figure 2.— Block of freshly blasted trap. 



This photograph of a block of freshly blasted trap (d) from the cen- 

 tral brecciated band shows above the hammer the coarse gab- 

 broid diabase (gd) and schlieren of the same. The one under the 

 hammer faulted. Below is a branching dike of the coarse gab- 

 broid diabase, having a thin central dike of holyokeite {h), which 

 is also faulted. Below the hammer the holyokeite is broader and 

 light colored. It is just above the h. At the right it is a wavy 

 white narrow band along the upper border of *the lower branch 

 of the coarse dike (see page 107). 



Plate 26. — Long Plumose Diabase ( X f ) 



The pyroxene crystals have shot out into flat blades, which branch with small angle 

 and are grouped in tufts all bent in a common direction by the 

 motion of the lava, as grass leaves bend in the wind. The central 

 band bends forward and so does not show its curvature (see 

 page 99). 



Plate 27. — Pyroxene, Magnetite, and Plagioclase 



Figure 1. — Flat-bladed pyroxene (natural size). 



This pyroxene is from the central portion of plate 26, showing the 

 central twinning suture, and the transverse basal parting (see 

 page 99). 



