ST. LAWRENCE COUNTY. 
357 
Equally interesting with the above, are the forms of calcareous spar ; and here, as at most 
other localities where a mineral substance is well developed, certain crystalline forms prevail 
to the exclusion of others. The primary faces are usually preserved, the modifications rarely 
proceeding so far as to obliterate them entirely. The angle most subject to alteration, is 
the obtuse solid angle, which is replaced by one plane inclining equally upon the adjacent 
faces. The other solid angles liable to replacement, are both the lateral and terminal edges, 
which are uniformly replaced by three planes resting on the primary faces. All these modi¬ 
fications are found present in the same crystal. But in all the crystals of the mine there is a 
tendency to the formation of twins : it is, in fact, more rare to find single crystals than twins ; 
and what is exceedingly interesting in crystallography, is that the plane which bisects the 
crystal diagonally, and along which it may be split, is visible, one half of the crystal lying 
upon one side and the other half upon the other, but in reversed positions. These forms, 
however, are not confined to this mine, but occur throughout this region, in most of the loca¬ 
lities where lime is found in a crystallized state. 
Besides the modifications already noticed, there is a form under twenty-four faces ; that is, 
each of the six primary faces of the crystal has standing upon it a low pyramid of four faces, 
striated parallel to the primary edges. It is one of the rarest forms yet met with in this vein. 
But it sometimes happens, that when the obtuse solid angle has been replaced, nature seems 
to have altered her mind, and after the plane is well formed, she has built it up so as to make 
the primary faces and angles complete. Her progressive steps in this case are seen in the 
sprinkling of the whole surface of the replaced or secondary face with crystals of sulphuret 
of copper or iron, and upon this surface thus covered, the form is completed. At least it has 
all the appearance of having been thus modified, and then completed in the order I have stated. 
I can conceive of noway or mode by which this deposit of crystals upon a plane surface could 
have been effected, unless that surface was truly a plane at the time of their deposit. This 
process, however, is not confined to a single plane, modified as stated above ; but it is not an 
unfrequent discovery to find all the primary planes covered with pyrites, and upon these planes, 
thus clearly made visible, an entire secondary form built up around and upon the primary 
faces. 
In these remarks, I speak of all the modifications as having been consecutive. Whether 
such will be deemed literally true and correct by crystallographers, I am unable to conjecture. 
The formation of crystals, however, in this vein, does appear often to have been effected by 
several successive steps ; and not only do additions to preexisting solids appear to have been 
made, but the matter is frequently of a different kind, and often of a different colour and 
lustre. The nucleus, for instance, of a crystal of carbonate of lime, is transparent; while 
around and investing it is a material which is quite opaque, but arranged in perfectly distinct 
planes. Perhaps there is nothing mysterious in this mode of building up crystals. It has, 
however, struck me that many real illustrations are furnished in the forms and changes here 
described as occurring by successive additions, in order to complete and finish the solids as 
we now find them ; or, in other words, these solids have been made as theory proposes, when 
we attempt to explain the mode in which one form is converted into another by the successive 
