1901-2.] J. G. Goodchild on Scottish Mineralogy. 
329 
waste, may be continually in process of completion. Whether this 
is so or not, the possibility of such a process being always in action 
deserves to be borne in mind, and especially when we are dealing 
with the genesis of the distinguishing minerals of the eruptive 
rocks — the felspars. 
In some few cases Albite, or a substance agreeing with it in 
composition, has arisen through changes which have operated upon 
some zeolitic minerals. A specimen of the so-called Erythrite, 
pseudomorphous after Prehnite, (316/44), is exhibited in the 
Scottish Mineral Collection, from Bowling, Dumbartonshire. 
Possibly other instances may be brought to light when the subject 
is further looked into. 
The Albite crystals in the Collection just mentioned, of which 
Albite. Fig. 1. — Orthographic projection on Z. Poly synthetically twinned 
sub-individuals, again twinned in accordance with the Albite Law. 
Forms: — Right positive hemiprism and its left analogue { 110 } ; 
together with the right and left liemi-brachyprisms { 130 } ; the basal 
pinacoid { 001 } ; the brachypinacoid { 010 } ; the right and left 
negative hemipyramids (ill, Il2, 113} ; and the negative macrodome 
there are many fine examples, do not appear to have been figured.* 
I therefore think it well to give the following examples here. 
Their registration numbers, by which the originals in the Museum 
may be readily identified, are fig. 1 , 316/22; fig. 2, 316/7a; and 
* After a careful and prolonged search through the Scottish Mineral 
Collection, I have only been able to find a very small percentage of any of 
the crystals which correspond to the figures in The Mineralogy of Scotland. 
fig. 3, 316/7. 
