ZINC ORES OF THE EDWARDS DISTRICT 25 



veinlike extensions which frequently connect adjacent masses 

 (plates 3, 4 and 5). In its typical development it is sharply con- 

 trasted with the pseudomorphous serpentine, though more often 

 the practical distinction between the two varieties is difficult. 



Cases of the molding of sulphides upon replacement serpentine 

 have not been frequently observed, but several occur that seem to 

 be of this nature (plate n, figure 2). In this type of serpentine, the 

 form is its own and not inherited from an older mineral, and hence, 

 if this form controls the deposition of the sulphides, it is evident 

 that they are younger than the serpentine. Even if the replacement 

 serpentine always starts with the alteration of diopside, if the 

 former spreads beyond the limits of the latter mineral, the form 

 becomes that of serpentine and plays the same role with reference 

 to sulphides that it would if no diopside entered into the process. 



This evidence then, so far as it goes, indicates that the sulphides 

 were actually molded upon, and are therefore younger than, ser- 

 pentine ; but, as a matter of fact, the blende in these cases is nearly 

 always of the very light-colored, younger type and thus is of no value 

 in this connection. Even in the rare cases of apparent molding of 

 early sulphides on replacement serpentine, the relation may be due 

 to replacement of the former by the latter, implying an age relation 

 just the revere of that suggested at first glance. 



Figure 2, plate 6 and figure I, plate 7 are of interest in this con- 

 nection. The molding of pyrite, in the former, and of blende, in 

 the latter, on the smoothly curved surface of grains of pseudo- 

 morphous serpentine is very marked, while equally marked is the 

 contrasting scalloped margin of the replacement serpentine, which 

 has been added to the pseudomorphous serpentine. Though not 

 shown in the photograph, the smooth curve of the latter is continued 

 inside of the replacing serpentine, making a clear line of demarka- 

 tion between them. So often are these relations repeated, that it 

 seems necessary to conclude that the blende was deposited upon the 

 rounded grains before the growth of the replacement serpentine. 

 This being the case, it is evident that the relations described give 

 no positive evidence as to the relative ages of blende and pseudo- 

 morphous serpentine, but, since both varieties of serpentine are the 

 product of essentially the same conditions, it is probable that they 

 are of the same age and younger than the dark blende. 



But while it is true that the actual space relations of the minerals 

 give no positive evidence of their relative ages, still the generally 

 rounded form of diopside in the limestone, the molding of sulphides 



