146 
Proceedings of the Eof/al Irish Academy. 
in our first memoir;* and others could be cited. But it would be some 
advantage to us to learn how such cases could be explained otherwise 
than by pseudomorphism en masse. 
We are quite aware that it is usual to regard pseudomorphs as mineral 
substances, replacing others, and retaining their crystalline form ; but 
we cannot believe that Blum, Bischof, and Dana, take this limited 
view. Examples do occur without any crystalline form remaining, 
especially when the change takes place in the external portions of the 
crystal ; and certain minerals (chondrodite) seldom or never give rise 
to pseudomorphs in the form of crystals ; while negative cases appear to 
be the absolute rule with serpentine. Now, such being the case with 
the latter mineral, how can we expect it to comport itself according to 
the general rule ? And, to be particidar, on what grounds are we to re- 
quire calcite to occur as a crystal-pseudomorph after serpentine, when 
the latter never presents a crystalline form proper to itself ? But, 
although such is the nature of serpentine, it seems to be quite over- 
looked that this mineral is common in the state of chrysotile, with a 
fibrous or asbestiform structure capable of being retained in the process 
of pseudomorphism. We have elsewhere made known examples of 
this variety, forming veins in ophite — parts of it here and there having 
been substituted by carbonate of lime, but still retaining the original 
fibrosity of the chrysotile. f Had nothing more than asbestiform car- 
bonate of lime been present in the veins, no one would have suspected 
it to be a pseudomorph after serpentine. 
If serpentine were an ordinary crystalline mineral, no doubt calcite 
would have occurred under the form of the crystal proper to it ; just as 
it is found in the form of dodecahedrons that were once garnets, — of 
oblique prisms that were originally orthoclase, albite, oligoclase, or 
augite ;% but usually occurring as an amorphous body, with the excep- 
tion of the allomorph or variety just named, and some others of the 
kind, its replacing mineral is prevented from assuming any other than 
the non-crystalline condition. From what has come under our obser- 
vations in the course of a prolonged investigation of the changes v/hich 
serpentine undergoes, we have every reason for believing that much — 
probably all — of the calcite immediately associated with it in ophitic 
rocks is its pseudomorphic replacing substance. 
It is simply from the rarity of non- crystalline cases that the rule 
has arisen of limiting the term pseudomorph to chemical replacements 
in the form of crystals. "We cannot, however, be far wrong, especially 
when there are precedents in our favour, in extending the name to 
similar phenomena in amorphous or rock masses. 
We may have " failed in showing," by our hypothesis, " why" ser- 
pentine has been replaced by calcite ; but we can confidently appeal 
* " Quarterly Journal Geological Society," vol. xxii., p. 216. 
t "Proceedings of the Eoyal Irish Academy," vol. x., pi. xliv., fig. 9, p. 531. 
X " Dana's Mineralogy," 5thed.,pp. 272, 344, 361 ; Bischof, vol. ii., p. 315. 
