130 Cross and Hillebrand— Minerals in the basalt of 



while the more perfect crystals are present in the large cavities, 

 and are usually small and numerous. 



In time i>i te follows anuleite. The opti- 



cal properties of this apophyllite are noteworthy in so much as 

 they seem to indicate very clearly the cause of the anomalous 

 action, so often noticed in that mineral. No hypothesis of a 

 complicated twin structure, such as that of Eumpf,* to cite the 

 most recent, as well as the clearest and ir~~ * ' 



explain the phenomena observed in this apophyllite. While 

 -tion of the observed anomalies is impossi- 

 ble in this notice, the chief features will be given. 



If from a small, clear crystal from Table Mountain a section 



be taken parallel to the base (0) and so situated that it cuts 



both pyramid (1) and dimetric prism (i-i), an eight-sided fig- 



ts (see fig. 1), in which the outlines of the pyramid 



will be referred to as those of the normal prism (7). 



Such a section seen between crossed Nicols, whose principal 

 sections lie parallel to the diagonals of /(position I), presents 

 ! an • >i by the diagram, fig. 1. 



There appears, namely, a dark square, whose 

 sides lie parallel to the outlines of the prism 

 (I), with dark lines running to the outlines of 

 ii, and perpendicular to the same, thus coin- 

 ciding with the diagonals of I. The dark 

 square and lines are well defined. The outer 

 w zone, divided by the dark lines into four seg- 



ments, is in position I at its maximum of brightness. 



On revolving the section through 45°, or until the prineip;il 

 sections of the Nicols coincide with diagonals of i-i (position II), 

 the whole field becomes equally dark, and the interference 

 cross of the calcite plate suffers no distortion in an} r part of the 



The dark square and lines revolve with the section, and the 

 darkening of the outer zone is gradual. It is in but few cases, 

 however, that the action of a crystal agrees entirely with the 

 above. The variations can, however, be most easily described 

 and best understood thai action as a basis. 



In the first place, the outline of the dark figure is seldom that 

 of a true square, its sides being usually more or less broken 

 lines, even when the angles remain nearly or quite 90°. It may, 

 too. resemble a rectangle rather than a square. The dark lines 

 subdividing the outer zone are less frequently variable than the 

 outlines of the figure. In proportion as the form of the dark 

 the square in regularity, so is the optical 

 behavior of the space enclosed uniform. In one case observed, 

 * J. Eumpf u Ueber den K tits » Min. und petr. Mitthel- 



TOgen von (i T« 69(1819). 



