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 ci-j'stallinc forms prevail 

 to the exclusion of others. The primary faces are usually preserved, the modifications rarely 

 proceedino- so far as to obhterate 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 crj'stals 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 througliout 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 crj'Stal 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 

 sprinkhng 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 eflfected, 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 tlie primary 

 faces. 



In these remarks, I speak of all the modifications as having been consecutive. Wliether 

 such will be deemed hterally true and correct by crystallographers, I am unable to conjecture. 

 The formation of crystals, however, in this vein, does appear often to have been efi"ected by 

 several successive steps ; and not only do additions to preexisting solids appear to have been 

 made, but the matter is frequently of a difierent 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 



