340 Proceedings of Royal Society of Edinburgh. [sess. 
exactly form a divisor of 360°. Hence the compound crystal 
built up by growth between adjacent individuals always shows 
re-entrant angles. I have never seen a crystal of Cerussite that 
did not show some trace of the twinning referred to. See figs. 
10-12, 14-16. 
One remarkable feature connected with these compound 
Fig. 15 . 
Cerussites is that the c face {001} is (or appears to be when 
studied under a high-power lens) coplanar to all the individuals, 
no matter what amount of irregularity of growth may characterise 
the other parts of the crystal. This feature will recall what was 
stated above in connection with Alhite. If these cases stood 
alone one might pass them by as accidental. But the same 
features are seen in the case of many other minerals, and they 
must be due to some cause which has not as yet been satisfactorily 
explained. One is almost tempted to regard it as a kind of 
