T. L. Walker — Examination of Triclinio Minerals. 183 



for thirty seconds with a half-and-half mixture of sulphuric and 

 hydrofluoric acids, very interesting figures were obtained. At 

 first sight there seems to be no regularity about them, but it is 

 soon seen that all the figures can be referred to a very simple 

 primitive corrosion pit which resembles in outline a right- 

 angled triangle whose sides are 3, 4 and 5, the shortest side 

 being parallel to the brachypinacoidal edge and the_ smallest 

 angle directed towards the left. The figure on 001 is simi- 

 lar and has its smallest angle pointing in the opposite direction. 

 Figure 16 represents a basal cleavage fragment from an albite 

 crystal simply twinned according to the albite law. _On the 

 left are the figures on 001 and on the right those on 001. The 

 primitive figures are seen to give, on further development, such 

 pits as those shown immediately below them. The arrow- 

 headed corrosion figures are the commonest and appear to be 

 the adult or final form. All intermediate stages in develop- 

 ment between these are plainly seen under the microscope. 

 The offshoot from the primitive form may take place from one 

 of the angles and produce nail-shaped figures. This can take 

 place from any of the three angles and can be traced through 

 all the intermediate stages. It is commonest from the right 

 angle of the triangle, as is the case in the instance figured. 



Since the primitive form gives rise to four different final 

 forms or types and a large number of intermediate stages, it is 

 not surprising that at first sight one should not see any defi- 

 nite order. The first, third and fourth types are the com- 

 monest. 



This development from simplicity and uniformity to variety 

 and complexity appears to be almost as common in crystal life 

 as biologists have shown it to be for organic life. Minute 

 crystals and tiny etching figures give promise of a heteroge- 

 neity and variety which they do not actually possess in their 

 earlier stages of development. 



But there is another form of etching figure to be noted. 

 Suppose a figure should begin exactly on the line of contact 

 between twinning lamellae, the figures would develop sym- 

 metrically on both sides of the line of contact. This is illus- 

 trated in figure 16. These figures possess a plane of sym- 

 metry and are such as we might expect on the basal plane 

 of a monoclinic crystal. If the albite twinning lamellae were 

 very, very thin, then all the figures would be like these, because 

 every figure would be large enough to extend across several 

 lamellae. This only proves by means of etching figures what 

 has been long known from geometrical crystallographic studies 

 — viz, that twinning increases the symmetry of a crystal. 



Some of the pits above described are represented in the 



