FENNER, THE WATCHVNG BASALT 163 



ther, and less of the original structure survives. The resulting forms 

 appear very complex at first and would be difficult of interpretation, if 

 clues had not been found in other sections. Plate XIII, fig. 3, illustrates 

 a typical portion. In the upper part, one large and two small diopside 

 crystals are seen, survivals of original phenocrysts of the vitrophyre, and 

 at various points there are remnants of small feldspar laths. Some parts 

 of the original glass are little altered and retain the brown or olive-^reen 

 color. In other places alteration has bleached the color or has formed 

 chloritic aggregates. 



The presence of much analcite is a prominent feature of slide 3. It 

 follows generally the vermicular courses of percolation, but in two places 

 it lies in broad bands between lines of ferruginous sand and would seem 

 to have filled open cracks of some width. A portion of one of these bands 

 appears in the upper right-hand portion of the figure. The width of this 

 band is about 0.3 mm. The other is broader, averaging 0.75 mm. The 

 analcite includes aggregates of light-colored plumose chlorite. A small 

 amount of non-isotropic zeolitic material is also present. 



Slide 61 (Plate XII, fig. 3) is another example of a shattered glass 

 whose angular fragments have become mingled with ferruginous sedi- 

 ment. The sedimentary material appears to be unaltered, whereas the 

 brecciated glass has been changed to albite, zeolites and chlorite. Among 

 the zeolites, analcite is prominent. There are also aggregates of hematite 

 scales and a few grains of chalcopyrite. The forms assumed in alteration 

 resemble those which have been described for other sections and are illus- 

 trated in the figure. 



Slide 60 (from the same hand specimen as the last) shows similar 

 phenomena. Analcite is exceptionally prominent, and other zeolites 

 almost lacking. Chlorite and serpentine are in notable quantity. Plate 

 XII, fig. 4, shows the characteristic structure. The banding assumed by 

 the secondary products, which forms a prominent feature in many of the 

 slides, is well brought out. The broad band which lies across the field a 

 little below the center consists of a middle portion of light-green chlorite 

 in rather large scales, bounded by darker and lighter bands of similar 

 material in finer scales, possibly with some serpentine. Beyond, on each 

 side, there is a sharply defined narrow band of colorless analcite, followed 

 in most places by a band of fibrous chlorite, and more analcite in broad 

 areas. The large circular form in the upper portion of the field is a 

 chlorite nodule, encircled by narrow bands of analcite and chlorite. A 

 small grain of chalcopyrite appears in the middle of the large band at the 

 intersection with the vertical cracks. 



