GENERAL DESCRIPTION OF THE SCHLIEREN 99 



solidification, } T et this transition was often very abrupt and was always 

 made within an inch. The coarse variety generally seemed like " schlie- 

 ren " in the finer, but also seemed at other places to have been fluid for 

 a slight^ 7 longer time, since small dike-like apophyses two or three inches 

 wide branched off from the coarser and penetrated the finer rock for 

 several feet, and fragments of the finer rock were sometimes slightly 

 separated from the rest and floated off into the coarser. The long blades 

 of the augite often rest by one end on the surface of the finer trap and 

 project into the coarser for an inch or more. It was as if some foreign 

 fluid had been introduced into the liquid lava when near solidification, 

 and had been stirred into it in irregular streaks by the internal motion 

 of the mass, which fluid made that part of the trap a little more easily 

 fusible, so that it remained liquid for a slightly longer time than the rest 

 and crystallized into coarser and peculiar types. There was thus a 

 transition from the schlieren to the dike condition. It was repeatedly 

 observed that several of these schlieren were placed one above another 

 and all about parallel with the upper surface of the trap sheet, as if their 

 shape was controlled by the advancing motion of the main trap sheet. 



VARIETIES OF THE SCHLIEREN ROCKS 



Long plumose diabase. — One of the most remarkable of the schlieren 

 rocks, which I have called the 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 feather-like pyroxenes have shot 

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

 wide (see plate 26 and plate 27, figure 1), which radiate in plumes like 

 a radiated actinolite. They branch at small angles and are bent grace- 

 fully or sharply twisted, as if they had shot out rapidly into the liquid 

 glass and had been swayed in its currents like 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 sand- 

 stone inclusions. This growth is essentially spherulitic, although the 

 sheaves form only a small portion of a sphere. 



There is a resemblance to the curved and blanching feldspar blades in 



XIV— Bull. Geol. Soc. Am., Vol. 16, 1904 



