20 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.66. 



are in stout prisms of about 3 mm. average length. There is no evidence of 

 granulation of any of the constituents, so the bending of the augite must be 

 attributed to disturbance during crystallization. A little iron ore occurs, and 

 moderately coarse micropegmatite interstices in small amount. The feldspar 

 of these could not be determined. Where micropegmatite is in contact with 

 iron ore and augite, secondary biotite has sometimes been built. The rock is a 

 gabbro near augite diorite. 



No definite relation of the gabbro to the sill boundaries could be made out. 

 There is usually a gradual passage from diabase to gabbro, but in some cases 

 small dikehke masses of the gabbro were found in diabase. The gabbro prob- 

 ably represents the more slowly crystallized, slightly more acid parts of the sills. 

 This phase is well developed in the area west of Logan Lake. In places in this 

 area the gabbro becomes very coarse, with pyroxenes up to 3 inches in length 

 often showing alignment, indicating motion of the mass during crystallization. 



The correspondence of the foregoing with the features of the Goose 

 Creek locahty is striking. Rocks having points of simiLarity which 

 occur in diabase of the Holyoke trap sheet of the Connecticut Valley 

 in Massachussetts have been described by Emerson.^ His description 

 of "long plumose diabase" may be quoted to show the similarity of 

 the augite in those phases to what has been described; although I do 

 not agree that the large size of the augites indicates rapid growth. 



One of the most remarkable of the schlieren rocks, which I have called long 

 plumose diabase, is found only in the immediate vicinity of the breccia band, 

 and contains filaments of the brightly rusting ankerite derived therefrom It is 

 a coarse-grained jet-black fresh-looking rock, in which the featherlike pyroxenes 

 have shot out in flat thin blades 3 or 4 inches long and nearly a fourth of an inch 

 wide which radiate in plumes like a radiated actinohte. They branch at small 

 angles and are bent gracefully or sharply twisted, as if they had shot out rap- 

 idly mto the liquid glass and had been swayed in its currents Hke a tuft of 

 grass leaves in the wind. A twinning plane runs down the center of each blade 

 and close set basal partings run at right angles to the same. These have the 

 effect of the midrib and pinnulae of a feather. The resemblance to grass is 

 greatly heightened because the rock has been fissured across this band and 

 many of the pyroxenes have, from weathering, turned a bright green, or even 

 straw color and white like dry grass. This is a change to talc. This variety 

 appears in perfection only in a narrow irregular band about 10 inches wide, 

 traceable several feet in the ledge near the band of sandstone inclusions. This 

 growth IS essentially spherulitic although the sheaves form only a small portion of 

 a sphere. 



The pyroxene is an almost colorless sahhte which is slightly blackened by 

 refusion at surface and along certain cleavage planes. The basal parting is very 

 marked, and this causes the feathery appearance. The central suture is caused 

 by twmning according to the usual law on (100), and the crystal is uniformly 

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

 obhquely through the thin plate, causing the broad central suture, which completes 

 the resemblance to a feather. The extinction is thus about 23 degrees obliquely 

 to right and left, and an optical axis appears in the border of the field. The 

 associated feldspar is labradorite, Aba An^. 



' Benjamin K. Emerson, Plumose diabase and palagonite from the Holyoke trap sheet. Bull Geol 

 Soc. Am., vol. 16, pp. 91-130, 1905. 



