BASIC VOLCANIOS OF HEMLOCK FORMATION. 133 



discernible reason, the iron carbonate (siderite) may replace the calcite, in 

 which case we get a dark bluish-black variety of matrix (p. 117). The 

 siderite masses do not differ essentially from the calcite, though in some of 

 them a A^ery small quantity of actinolite is found associated with the chlorite. 

 As illustrating the purely local development of these two carbonates, I 

 would mention having observed in one section a band of siderite separated 

 from a band of carbonate, which from its color appeared to be quite pure 

 calcite. One may also see commonly in exposures areas of pure white 

 calcite, almost in juxtaposition with areas of siderite. 



It is a fact generally recognized that carbonation is a process confined 

 to the outer crust of the earth, so that we may perhaps best explain the 

 local occurrence of these carbonates replacing the basalt as products of 

 carbonate-bearing waters. That such carbonation of the igneous rocks 

 through which these waters percolate is now taking place seems certain. 

 The carbonate grains in the rocks described are shattered and elongated, 

 or at least show undulatory extinction. They thus give evidence of having 

 been more or less mashed since their production, and this mashing probably 

 took place after they had l^een more or less deeply buried, and was, as a 

 matter of fact, to some extent due to the pressure of the superincumbent rocks. 



The probability that these rocks have been thus deeply buried subse- 

 quent to their formation is to be borne in mind with special reference to the 

 next process to which they have been subjected, that of silicification. This 

 process is most clearly shown in the siderites, and the phases of alteration 

 noted in their study will be briefly described. 



The microscope shows the siderite matrix to be a coarsely granular 

 aggregate composed essentially of crushed siderite grains. Between these 

 grains in a few places are small grains of quartz, flakes of chlorite, and very 

 rarely needles of actinolite. Large quantities of black ferruginous specks 

 are included in and also lie between the quartz grains, and such specks are 

 also to be seen included in siderite areas, but close inspection shows that 

 they are also associated with blebs of quartz. The chlorite flakes and 

 quartz grains are generally elongated in the same direction, and the quartz 

 shows wavy extinction. 



A more advanced stage in the process of silicification was studied in 

 the case of a rock which is bluish-black in color, exceedingly fine grained, 

 and minutely schistose, the schistosity agreeing with the contours of the 



