106 B. K. EMERSON — PLUMOSE DIABASE AND PALAGONITE 



tions). The glass many times molds perfectly the minute crystal ends 

 of the calcite and the quartz and always incloses them entirely, and there 

 can be no doubt that they have crystallized out from the liquid magma 

 in quick succession, and the calcite has always crystallized first. The 

 presence of calcite and quartz in solution together in the magma perhaps 

 presents difficulties. It can only be said that the proof of the fact seems 

 complete, and that at a pressure which would prevent the breaking up 

 of the CaCo 3 molecule, it and quartz may well coexist. 



The glass also contains minute lithophysse with crumpled inner sur- 

 faces often filled with sphserocrystals of ankerite and quartz. All the 

 forms enumerated in this section are described in some detail, in connec- 

 tion with the theoretical discussion as to their origin, on pages 117. 



THE LITHOIDAL OR HOLYOKEITE BASE 



There appears around the glass clots or groups of clots a broad bluish 

 halo of very fine grained base, which seems at times purely quartzose or 

 chalcedonic — at times felsitic — always compact and aphanitic. It is 

 sometimes clear blue, but more often bluish black to black. The larger 

 phenocrysts are often excluded from this halo for a considerable space. 



This base is found with the microscope to be extensively developed as 

 a kind of mesostasis in small isolated angular patches in the plumose 

 and coarse ophitic types, and making up the whole ground in the coarse 

 porphyritic quartz diabase included in and between the great feathery 

 feldspars. It is an interesting case of micro-differentiation. 



Where the interstitial areas are bounded by feldspars the glass clots 

 tend to be central. Where the boundary is partly pyroxene the glass is 

 apt to be adjacent to it, as if a differentiation had set up in the original 

 interstitial magma (caused by an influence of the already formed feld- 

 spar surfaces like that which causes albite to coat the surface of micro- 

 cline), by which the albite-quartz hyalopilitic ground grew outwardly 

 from the feldspars, rejecting and crowding away the black basic iron- 

 magnesium glass. Under the microscope it is a colorless base or is pale 

 brown from a great quantity of brown dust, sometimes aggregated in 

 fusiform shapes. With polarized light a beautiful and peculiar hyalopi- 

 litic structure appears (see plate 30, figures 3 and 4). The delicate satiny 

 needles of plagioclase radiate from central angular or shapeless albite 

 grains, and are often arranged in very beautiful and characteristic loose 

 anastomosing groups like hoar frost, the interspaces being filled with a 

 mosaic of shapeless and blending grains. With common light these 

 grains are indistinctly fine fibrous ; with polarized light they seem to be 

 albite filled with minute quartz rods, forming an exceedingly fine and 

 close set micropegmatitic structure. This ground passes into radiate- 



