328 Proceedings of Royal Society of Edinburgh. [sess. 
augen (as well as the minerals associated with them). This appears 
to have resulted in a slight separation of the polysynthetically- 
twinned laminae, whereby a peculiar chatoyance has been developed. 
Excellent examples of this structure may be seen in the Scottish 
Mineral Collection in the Edinburgh Museum of Science and Art. 
The subject is referred to here as one illustration that may be 
chosen out of many in which a characteristic lustre has been 
developed by the formation of structures allied to the twinning 
referred to. As examples, reference may be made to Stilbite, 
Heulandite, Cerussite, and many others. In all of these closely- 
adpressed laminae occur as original features. In other cases a slight 
separation along the cleavage planes has arisen through the opera- 
tion of changes subsequent to the completion of the crystal growth, 
as in the case of some Pyroxenes. It may be as well to remark 
that polysynthetic twinning in Albite cannot well have arisen 
through causes of a mechanical nature operating since the crystal 
was completed. 
Albite has lately been shown by Mr Clough of the 
Geological Survey * to occur as a constituent of certain schistose 
rocks in Cowal. There is clear proof that the rocks in question 
were originally sediments of the normal kind, and consequently 
there has been much discussion as to how the Albite got there. I 
should like to offer the suggestion that the Albite may, possibly, 
have originated through the transference, by osmosis, of sodium, 
from sea water, while these rocks were undergoing the dynamic 
changes which have led to their present schistose character. 
Geologists are apt to overlook the fact that there is a continual 
transfer of the alkaline carbonates from the land to the sea, and 
that the proportion of these returned to the land in the form of 
wind-borne sea-spray cannot bear more than a small proportion to 
that of the quantity travelling in the opposite direction. If part of 
the alkali is returned to the sediments beneath the ocean floor by 
osmosis or other causes, one can readily understand how the 
balance is maintained, and also how the cycle of change from eruptive 
rocks to sediments, and from sediments back again, by the restora- 
tion of the alkalies divorced from their parent rocks by sub-aerial 
* Geological Survey Memoirs in The Geology of Cowal, by C. T. Clough 
(and others). 
