PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 425 



Diabase.] 



minerals shall be made we prefer to keep it distinct both from prehnite and from 

 thomsonite. With the latter it is allied in the position of its axial plane, but this 

 fact alone is not sufficient to destroy its distinctness from thomsonite, for it is found 

 also in other zeolites, such as mesolite and sometimes in laumontite. One section. 

 Age. Manitou. N. H. w. 



No. 567. DIABASE (with olivine}. 



From Lucky bay, south side of Isle Royale. (Lucky bay is represented on plate III, Tenth Annual Report.) 

 Rcf, Annual Report, x, page 53. 



MI-IJ. A porphyritic trap-rock, specked with a green radiated mineral and with 

 brown, and apparently with epidote. 



Mic. The feldspar is triclinic, but permeated with decay. Zoixitr particles are 

 scattered through it, and a little calcite. The zoisite has low birefringence; indeed, 

 shows only gray and white colors. It is thus distinguished from epidote, which is 

 abundant in the slide, and sometimes is embraced in the feldspar. A little chlorite 

 is also embraced in these decayed feldspars. 



(Jiutrtz appears as a product of alteration. 



I'".l>ii/nle is abundant, both fibrous and regularly cleaved, in independent crystals, . 

 the former being the more refractive. It does not show the "intense polychroism" 

 which is characteristic of the Sulzbach crystals, but rather a steady straw-yellow 

 color, which rarely disappears, although it fades on rotation. The spheruliths are 

 irregular, and are rather more like patches of curved or distorted crystals, showing 

 between crossed nicols a transition from light yellow upward to orange yellow, red, blue, 

 green, as the point of observation passes from the periphery of a spherulitic mass to 

 the centre, thus changing from parallelism to perpendicularity with the fibration. 

 This also indicates that the axial plane is transverse to the fibration. 



Olivine has almost entirely given place to a finely fibrous "serpentine," or 

 thalite which is almost isotropic. 



A/nifife remains in its original crystals, and is abundant. It is in the titanite, 

 the epidote and the plagioclase. The crystals are sharp and perfect, frequently 

 showing hexagonal sections. They are surrounded by a coating of hematite, or of 

 limonite. 



Chlorastfolite is closely associated with some of the epidote, forming radiated 

 nests similar to the description given under the last number, surrounded generally 

 by epidote. 



An occasional brightly polarizing needle, seen in the epidote grains, remains 

 undetermined. 



Untile, more or less altered by stains of hematite, and apparently changed to 

 leucoxene, forms some conspicuous masses. Some of these grains are so large that 



