

PROFESSOR HEDDLE ON THE RHOMBOHEDRAL CARBONATES. 511 
bulging sides, as if there had been some expansion; but they are, in the case of 
the two latter at least, markedly porous. 
I have called the replacing substance orthoclase or albite—it has not been 
sufficiently determined. The substance of some of the red pseudomorphs of 
stilbite was many years ago analysed by myself, and found to be albite; the 
analysis appeared in the “ British Minerals” of Grea and LeErtsom, under the 
name of weissigite. The erythrite of THomson (?oligoclase) would appear to be the 
material of others. The material of Laumonite pseudos was found by Biscyor 
to be orthoclase; and the cluthalite of THomson, physically ill-defined, may be 
but a badly selected or badly picked specimen of one or other of these. 
Prehnite after Laumonite. 
6-86 6-61 
or Thomsonite. 
2( (3CaO , }Na,O) SiO, + A1,0,,Si0,) +5H,0 
277 
651 + 2:35 —- ag = 6-67 
Possibly both of those pseudomorphs occur; they have no terminations to 
their crystals, and are altogether so much of the nature of skeletons that it is 
not easy to determine the mineral replaced. No weight either way can be 
attached to any specimen I have seen of these. 
Still one marked exception remains to be noted— 
Chalcedony after Datholite (Haytorite). 
SiO, (CaO , B,O, + CaO, 28i0,) + H,O 
60 + 2-65 - = = 7-66 320 + 2-9 — ee = 6.13, 
The crystals of Haytorite are usually very hollow; the freely radiating 
crystals of quartz—for they are more of the nature of quartz than chalcedony— 
of which they are formed, have so much the appearance of having grown in a 
thoroughly empty cavity, that probably this does not come into the category 
of chemical replacements. 
We are, therefore, in our exceptions, reduced to the cases of the two felspars 
replacing the zeolites ; and, as has been shown, doubt attaches also to them. 
So far then as the specimens in the writer’s cabinet throw light on the ques- 
tion, no well-marked or indubitable exception to the law previously propounded 
exists. 
