1894. PROCEEDINGS OF THE NATIONAL MUSEUM. 647 



frequently macroscopically evident on a freshly broken or a polislied 

 surface, appearing as dull red areas 1 to 2 mm. in diameter, surrounded 

 by a narrow border of the white calcite. In none of the many sections 

 examined am I able to find even a trace of unaltered olivine. 



The feldspars of the groundmass are, in part at least, a plagioclase 

 variety, as shown by the numerous twin striae. There are, however, 

 abundant clear glassy forms appearing in the section in the form of 

 stout rectangular areas, which in some cases give extinction angles 

 exactly parallel and in others inclined a few degrees from the axis of 

 elongations. These are assumed to be orthoclase, an assumption 

 apparently borne out by the large percentage of potash shown in the 

 analysis. The augites of the groundmass have the same color as the 

 porphyritic forms, but occur in idiomorphic, and also in imperfect, and 

 often sharply wedge-shaped and angular forms filling the interstices of 

 the feldspars. The brownish mica occurs only in small and very irreg- 

 ular shreds associated with secondary chloritic material. 



Approaching the line of contact the groundmass becomes more dense, 

 but still retains its largely crystalline character. The porphyritic 

 augites here are of a light sage-green color and show very perfect crys- 

 tal outlines. They are, however, much more decomposed than those in 

 the coarser and less compact portion, presenting a mass of rounded and 

 angular pale augite fragments, intersi)ersed with calcite, iron oxides and 

 undeterminable decomposition products of a dirty white color. At the 

 immediate line of contact with both over and underlying shales there 

 is a narrow band, from 3 to 6 mm. in width, of a brownish color, consist- 

 ing of the augites and feldspars of the first generation imbedded in a 

 wholly or partially devitrified base, which remains always light between 

 crossed nicols, and shows a mass of illy defined rounded and elongated 

 globules, over which play imperfect and distorted black crosses as the 

 stage is revolved. Both the feldspars and augites are here replaced by 

 calcite pseudomorphs. The shale itself is strongly injected with calcite 

 for the distance of a few millimeters from the line of actual contact. 

 The line of separation between the shales and eruptive rock is in all 

 cases perfectly sharp, the fused material having flowed over and around 

 the particles of quartz and feldspar in a manner implying a high degree 

 of fluidity. Contact metamorphism of even so large a mass injected in 

 a highly fluid condition, and cooling so slowly as to become almost holo- 

 crystalline, is here reduced almost to a minimum, owing to the refrac- 

 tory nature of the materials of which the shales are composed. Thin 

 sections show these to be made up of small fragments of quartz and feld- 

 spar with but a small amount of intersticial space now occui^ied by 

 secondary silica having the same crystallographic orientation as the 

 adjacent quartz granules, and by very minute, needle-like flecks of sil- 

 very white mica, evidently developed from the small amount of original 

 amorphous cement. The shale is, therefore, no longer at this point an 

 agglomerate of fragments adhering by means of an amorphous cement, 



