THE CHESTER AMPHIBOLITE AND SERPENTINES. 107 



The interior is a dark oil-green, Hecked with white or yellow; opaque 

 black when wet, and with ])eculiar ji^reasy luster reflected from large 

 cleavage surfaces, which run through the wlioii- mass. These smfaces reach 

 a size of 20 x 40™'", have a pearly luster, and are, in many cases where the 

 rock is deeplv weathered, bleached to an isabella-yellow or changed to 

 a white mass like kaolin. They are covered with an acute-angled network 

 from a second cleavage, like that of hornblende, but more acute, and a 

 satiny sheen runs over the face from the presence of fine treniolite needles, 

 arranged parallel to this cleavage and gradually encroaching njjon the 

 original mineral, which proves upon microscopical study to be sahlite. 

 Slices cut parallel to the perfect parting (see fig. 1, PI. II), which proves 

 to be P (ODD, show a fine, regular network of tremolite needles, which 

 polarize with an obli(iuity of about 15° and coincide in position w^ith the 

 acute cleavage of the original mineral mentioned above. Where this 

 secondary tremolite has not come to occupy the whole space the meshes 

 are occupied by a colorless sahlite, showing in traces an inteiTupted pris- 

 matic cleavage and a delicate lineation parallel to go P co (100), with traces 

 of a second at right angles to this. In the figure this fine lineation is of 

 necessity too coarsely represented, it being visible only with high power, 

 and it is given specially to show the extent of the unchanged sahlite. The 

 latter polarizes with extreme brilliancy, and characteristic sudden changes of 

 tint appear over its surface, arising from the brittleness of the thin laminse 

 due to the very easy parting on P (001), which renders it diflicult to polish 

 it to a true surface. 



The mineral is positive, and the optical axial plane is at right angles 

 to the fine lineation— i. e., is in oo P » (010)— and a single axis appears, 

 and this plane bisects the acute angle of the tremolite network, which meas- 

 ixres about 54^. This would make the cleavage, which has determined the 

 position of the tremolite fibers, approximate to qo P 2 (120), the counterpart 

 of cc P 2 (210)— the prism of hornblende when reckoned upon the pyroxene 

 axes. An examination of the Bolton (Massachusetts) sahlite shows that dis- 

 tinct traces of the same cleavage existed in fresh specimens. Here, as is not 

 unusual, it is rendered nuich more distinct in the process of decomposition. 

 In slices cut at right angles to the perfect basal cleavage or parting 

 the strong equidistant lines of separation are the marked feature, and these 

 lines quite far apart are the seat of most advanced change. 



