BASIC VOLCANICS OF HEMLOCK FORMATION. 131 



tiou.. It may be characteristic also of basalts with no ellipsoidal parting, 

 but it has been possible to follow the successive changes only in the ellip- 

 soidal basalts. This is due to the fact that each ellipsoid shows all stages 

 from the comparatively fresh material of the center to the much altered 

 material on the periphery, and to the most altered Ijasaltic material forming 

 the so-called matrix surrounding the ellipsoidal bodies (p. 114). 



The freshest part of the interior of an elHpsoid from this occurrence is 

 a very fine-grained micro-amygdaloidal basalt, in which in ordinary light 

 lath-shaped feldspar microlites can be readily distinguished. Upon close 

 examination the feldspars are found to be much altered, and in many cases 

 their crystal outlines are almost completely filled out by grains of calcite 

 and flakes of sericite and chlorite in a quartz-albite (!) aggregate. The 

 spaces between the feldspar laths are now occupied by large crystals of 

 epidote-zoisite, grains of iron oxide, a few flakes of chlorite, and innumera- 

 able small round yellowish-brown and greenish indeterminable bodies. The 

 epidote-zoisite crystals also include large quantities of the brown and green 

 globular bodies, showing that they were produced previous to the epidote- 

 zoisite. The substance in which this aggregate is embedded could not be 

 determined, as the aggregate is either so dense that nothing could be dis- 

 cerned or else underlain by feldspar. In the last case the substance is Loen 

 to be clear white. The minerals mentioned, with the exception possibly of 

 the iron oxide, have evidently been produced secondarily from the sub- 

 stance or substances originally filling the spaces between the feldspars. 

 Nothing points toward the original substance or substances having been 

 crystallized, andl am inclined to believe that it was glass. 



Toward the exterior of the ellipsoid the rock is more altered. The 

 zoisite and calcite are more abundant. The calcite occurs in the spaces 

 between the feldspars, as well as occupying parts of their outlines. (Figs. 

 A and B, PI. XXXI.) All of the other products drop into the back- 

 ground, owing to the fact of nouproduction, or concealment by the zoisite- 

 calcite aggregates. 



Still nearer the exterior of the ellipsoid the calcite frequently fills the 

 spaces once occupied by the feldspars with long scalenohedi'al crystals, 

 which in a way maintain the original igneous structure. The calcite is, 

 however, not confined to these feldspar areas alone, but, as stated above, 

 also occurs between them. 



