58 Scientific Proceedings, Royal Dublin Society. 



"well-defined hexagonal prism, close to its junction with the ortho- 

 clase, but well within the hexagon, and at right angles to the 

 principal axis of the hexagon. 



In this section the phenomena are so eloquent as to set the 

 question at rest. The attack is, in a word, seen at a much ad- 

 vanced stage. The beryl has broken down completely. Uni- 

 formity of extinction, which here, if normal, should be persistent 

 between crossed nicols, is no longer seen. Here and there hexa- 

 gonal forms, left standing by the invading orthoclase, remain quite 

 true to their original position, though veined and worn. These 

 behave as amorphous, save for the cross-hatch marks. Other 

 hexagonal outlines, with angles projected out of 120°, partially 

 restore illumination as they are rotated between the crossed nicols. 

 In short, patches of beryl are found, fallen in the fight, and cut 

 at such various angles with the optic axis, that they can hardly 

 be differentiated by colour or extinction — on the one hand from 

 beryl cut at right angles to that axis, and on the other from 

 beryl cut along that axis. 



Fig. 2, Plate n. (x 18 diameters), presents a remarkable pic. 

 ture of dismemberment and solution. The large, broken, and 

 incomplete hexagonal outline there shown was on the alteration 

 hypothesis originally a homogeneous portion of the parent crystal. 

 It is now girdled round with felspar, and broken up. Its cracks 

 are in continuity from side to side. It was even attacked and 

 veined by a primary inroad of felspar before the final attack eat 

 out a path, severing the primary vein and parting the mass. 

 Islands of beryl left standing, or borne down from its banks, 

 mark the course of this felspar flood. More than this, so com- 

 plete has the final solution been, and so simultaneous all round, 

 that movement of the dismembered hexagon after its isolation is 

 apparent. Thus it will be seen that the edge a is no longer 

 parallel to the edge d. It is, in fact, according to measurement, 

 about 14° removed from parallelism. This measurement was 

 taken on the photograph; it is then independent of the readings 

 of the angles of the hexagon. Placing now the cross wires of the 

 microscope along the edge b, and along the edge c, an angle of 

 132°, about, is scaled on the section. This is fairly concordant with 

 the observation made on the photograph. It should read 134°, to 

 agree with it. There is then evidence of movement of the detached 

 fragments relatively to each other. 



