AHT. i!8 MIXERALOGY OF TEIASSIC LIMESTONE SHANNON 5 



prisms and the feldspar in elongated laths. The rock is all more or 

 less affected by alteration and the feldspars are so sericitized that 

 their determination is impossible. The lath-like habit of the feld- 

 spars gives the rock an ophitic appearance but the crystallization of 

 the pyroxene and feldspar was apparently nearly simultaneous. Ir- 

 regular or rounded rather large dark spots in the section are appar- 

 ently aggregates of very minute grains of magnetite, dense in the 

 center and thinning toward the borders of the spot. Wliere small un- 

 sericitized remnants of the feldspar remain they have a refractive 

 index distinctly above that of Canada balsam showing that they have 

 not been albitized. In addition to the fine crystalline fabric which 

 forms the body of the rock, there are visible in thin sections certain 

 scattered areas, much larger than the average grain of the rock, which 

 are now green serpentine clearly secondary after original olivine. 

 Occasionally they inclose a core of unaltered olivine. These olivine 

 pseudomorphs seldom show complete crystal outline but have the 

 appearance of fragments of broken up larger crystals. Colorless and 

 fresh p3'roxene also occurs rarely like the olivine as larger isolated 

 crystals or groups of several crystals. The freshest rock is cut by 

 very thin cracks filled with fibrous, colorless serpentine. 



At the borders of the dikes there are chilled glassy phases which 

 have the same purplish color as the body of the rock except where 

 hydrothermally altered to a dull green. The glassy material has a 

 waxy luster and faintly conchoidal fracture. It is clear isotropic 

 glass of dark brown color in thin section and, like the crystalline 

 rock, contains scattered talc and serpentine pseudomorphs after 

 olivine and a few pj'roxenes. The isotropic glass grades into bire- 

 fracting material and at a distance of 16 millimeters from the con- 

 tact in one specimen, had graded into wholly birefracting very fine 

 grained material having a fibrous structure suggesting the structure 

 of the crystallized basalt. 



HYDROTHERMAL ALTERATION OF THE BASALT 



The alteration of the limestone, which, in the vicinity of the 

 basalt dikes is largely converted to lime-silicate rock, by the action 

 of thermal solutions is believed in great part to be subsequent to the 

 intrusion of the basalt. The solutions might naturally be expected 

 to exert some profound influence on the shattered basalt while pro- 

 ducing such drastic changes in the limestone but such is not the 

 case. All of the feldspar of the basalt is sericitized, and, as has been 

 mentioned, it is extensively traversed by narrow seams of serpentine. 

 Moreover, there occur, here and there, scattered in the diopsidized 

 limestone, small masses and fragments of more or less glassy basalt 

 from the dikes. These have lost their original purple color and are 



