BASIC VOLCANICS OF HEMLOCK FORMATION. 101 



from its alteration, and as it effervesces quite readily with cold HCl, it is 

 supposed to be ferruginous calcite. 



Sericite is found in minute flakes replacing the feldspars, and it is also 

 found in large porphyritic plates occurring in the eruptive groundmaps asso- 

 ciated with the porphyritic carbonate above described. In some cases we 

 find epidote in these altered basalts, in others zoisite. In a great number 

 of instances the same individual exhibits the high interference colors of 

 epidote and the low blue interference color of zoisite in different parts. 

 These different portions, formed respectively of the epidote and zoisite mole- 

 cules, are most closely intergrown, and I have therefore used the compound 

 term "epidote-zoisite," indicating this fact. Associated with this, one finds 

 in many of the specimens small mineral aggregates which merit somewhat 

 further notice. These aggregates have a brownish-yellow color and possess 

 a very high single and also a high double refraction. In these masses the 

 single and double refractions of the granules composing the aggregates 

 appear to be higher than that of epidote. In shape the aggregates vary 

 from perfectl)' round, zonally arranged spheres and irregular, elongated, 

 rounded aggregates to forms giving oblique quadratic sections. All of these 

 aggregates are found at times included in the epidote-zoisite crystals. In a 

 few cases the oblique quadratic sections were seen to occupy the centers of the 

 epidote-zoisite crystals, having' exactly identical outlines. It is believed that 

 they are composed of an epidote nuicli richer in iron than the common variety 

 with which they are associated. This increase in iron explains the darker 

 color and the increase in single and double refraction, as shown by Forbes. ■' 

 Why it should appear, especially in the aggregates, can not be explained. 



The chlorite does not appear to be entirely an alteration product of the 

 secondary hornblende with which it is associated. There is usually rather 

 more chlorite than it would seem could possibly have been formed from the 

 alteration of the hornblende alone. In some of the rocks the larger angles, 

 as well as the extremely fine areas between adjacent feldspars, are occupied 

 by a very fine felt-like chloritic mass. The chlorite which is not secondary 

 after hornblende is considered as the product of an altered glassy base. 



No glass was observed in the nonporphyritic basalts occurring in large 

 masses, but in one of the fragments of basalt in a tuff a dark chocolate- 

 brown glass forms the matrix in which are lying well-developed plagioclase 



' Epidote and its optical properties, by E. H. Forbes ; Am. Jour. Sci., 4th ser., Vol. 1, 1896, p. 30. 



