332 Proceedings of Royal Society of Edinburgh. [sess. 
here that the first number in the registration mark is that adopted 
for the species by Dana, in the Sixth Edition of his System of 
Mineralogy. It would be extremely convenient if, now that the 
numbers have been so used, mineralogists in general, including 
the author of the above named work — the most generally used of 
all — were to agree to continue the practice. It often saves much 
time to denote a mineral by its number. 
It may be well here to recapitulate the nature of the parallel 
between the minerals thus developed by metamorphic- action with- 
in marbles and those which constitute the commoner minerals of 
the eruptive rocks. Quartz is common to both ; Tremolite is a 
mere variety of Amphibole ; Diopside is a variety of Augite ; 
Andesine runs parallel, as it were, to Labradorite, the commonest 
felspar of the more basic eruptive rocks; Phlogopite is a near 
ally of Biotite ; Spinel may he regarded as representing Magnetite ; 
and, lastly, Forsterite is regarded as one of the Olivine Group. 
These facts need to be steadily borne in mind in all considerations 
respecting the origin of the eruptive rocks ; especially if we regard 
all these latter as simply having crystallised from aqueous solutions 
which were at a high temperature. 
It is, however, an interesting fact in this connection that some 
closely-allied minerals have evidently been formed under quite 
different conditions. For example, Pectolite, which in origin is 
almost to be regarded as a Zeolite, has certainly been formed from 
the decomposition of parts of an eruptive rock of basic composi- 
