BASIC VOLOANICS OF HEMLOCK FORMATION. 135 



agencies leacli out the remaining siderite areas and leave tlae thin fihns of 

 silica which lie between them standing wp, thus giving the rock the appear- 

 ance of a very dark pumice. As the silicification progresses the siderite is 

 very much reduced in quantity, the intervening siliceous areas increasing 

 correspondingly. The pressure exerted upon the rock has caused the 

 isolated siderite areas to take on an oval character, tile longer axes in 

 general agreeing and being perpendicular to the pressure. When such 

 siderite areas are leached out, the silica bands remain, and pseudo-amyg- 

 daloidal cavities are produced, giving a very perfect pseudo-amygdaloidal 

 structure to the hand specimen. This is the origin of that character of 

 matrix which some of the geologists have described in their field notes 

 as like rotten, worm-eaten wood (fig. B, PL XXVII). 



Although at present the material between the ellipsoids differs so 

 markedly from the rock forming the ellipsoids themselves, nevertheless 

 there is no reason for supposing the original composition of that part of 

 the rock mass to have been essentially different. The change in the 

 character of the basalt in passing from the ellipsoids toward the schistose 

 matrix is in mineralogical character much as has been described for other 

 basalts from this same district. The reason for the more complete degree 

 of the replacement process in passing away from the ellipsoids may be 

 readily understood from the discussion of the origin of the ellipsoidal 

 parting of the basalts, where the conclusion was reached that the matrix 

 between the ellipsoids resulted from the comminution of basaltic material 

 of the same general character as that of the ellipsoids. This matrix was ot 

 course more porous and probably more vitreous than the basalt, and hence 

 more liable to be altered. 



PYROCLASTICS. 



The majority of the clastic rocks have been derived from the basic 

 volcanic rocks already described. These elastics are very characteristic of 

 the Hemlock formation and constitute the greater part of it. They 

 comprise several classes, the more important of which are the eruptive 

 breccias, volcanic sedimentary rocks, and schistose pyroclastics. 



ERUPTIVE BRECCIA. 



The term "eruptive breccia" is here used to include those clastic rocks 

 in which angular fragments of an igneous rock are surrounded by a matrix 



