100 B. K. EMERSON — PLUMOSE DIABASE AND PALAGONITE 



the spherulites of Obsidian cliff. Professor Iddings cites Lehmann's 

 explanation of unequal surface tension for the curved forms. 



The pyroxene is an almost colorless sahlite, which is slightly black- 

 ened by refusion at surface and along certain cleavage planes, and this 

 causes the black color. The basal parting is very marked, and this 

 causes the feathery appearance. The central section is caused by twin- 

 ning according to the usual law on (100), and the crystal is uniforml}' 

 flattened on two of the prism faces (110), so that the twinning plane 

 passes obliquely through the thin plate, causing the broad central suture, 

 which completes the resemblance to a feather. The extinction is there- 

 fore about 23 degrees obliquely to right and left, and an optical axis 

 appears in the border of the field. The photograph (see plate 26) shows 

 imperfectly the length and curved character of the blades, which are 

 frequently notched along their sides by faces of the unit form. The 

 figure (plate 27, figure 1) shows much better the feather-like texture. 

 The associated feldspar is labradorite (ab 3 an 4 ), with extinction 30 de- 

 grees on (100). 



This variety, because of its proximity to the breccia band, has, like it, 

 its feldspars sometimes almost completely changed to a radiate tufted 

 mica, probably paragonite, whose fibers are arranged in its cleavages. 

 They are optically positive. Its pyroxenes are loaded with black gran- 

 ules along the parting planes. 



There is no glass in the brecciated band, and here glass particles are 

 rare and completely altered. They are often minute, hollow, and beauti- 

 fully botryoidal lithophysse, and the cavities are lined with a coarse radi- 

 ating, brightly polarizing devitrification product, which is sometimes 

 coated b}^ a la} 7 er of secondary calcite. 



Short plumose variety with remelted pyroxenes. — Another striking variant, 

 which may be called the short plumose form, is very fresh in appearance, 

 jet black, and coarse grained, and the abundant pyroxenes are in short 

 blades, twinned like those described above, but only about an inch long. 

 The peculiarity of this type of the trap is that all the large pyroxenes 

 are more or less remelted while retaining, wholly or partly, their original 

 positions and traces of original structure. This is shown in plate 27, 

 figure 2. The common pale brown pyroxene (see h in the right central 

 portion of the figure) has the strong prismatic cleavage (horizontal in the 

 figure) and a trace of the nearly vertical basal parting. Below this a 

 broad dark band g runs down athwart the figure, flanked on either side by 

 a lighter brown band dg. These are glass from the melting of the pyrox- 

 ene. In the zone of transition between the glass and the pyroxene the 

 basal parting is developed into an almost micaceous cleavage, and this 



