NOTE OX THE COlx'AE KOCK OF FFXakt^TI. 391 



Secondly, M'e have to consider the j)rodiiction of dolomite hy the alteration ol 

 calcite or aragonite. This subject has attracted my attention for many years, and, 

 though the fact of such a change admits of no doubt, I have never been able to 

 prove it experimentally. By keeping for a good many days a portion of Iceland 

 spar in a solution of chloride of magnesium, at a temperature much above the boiling 

 point under atmospheric pressure, I was able to replace carbonate of lime by carbonate 

 of magnesia in an external layer which easily scaled off and was proved to be almost 

 free from lime, but the process went on so slowly that it would probably have taken 

 years to have completely replaced the Iceland spar to the centre, as I have been able 

 to do in the case of some other artificial pseudomorphs. What the limit of tempera- 

 ture needed for this replacement may be, I cannot say, but, judging from what occurs 

 w^hen Iceland spar is artificially changed into a pseudomorph of carbonate of iron, 

 an alteration wdiich takes place in a few days at a temperature of 300° F., took place 

 to some extent at the normal temperature when the experiment was continued for 

 35 years, but judging from what occurred, it would probably have required some 

 hundreds of years to have produced a solid pseudomorph right to the centre. If I 

 had foreseen this, I should have started many similar experiments when I was at 

 such an age that there was a possibility of seeing a decided result. It must, how^ever, 

 be borne in mind that in the above-named experiment true dolomite w^as not formed, 

 and the tvhole of the lime was replaced, instead of nearly one-half. It is possible that 

 there is a reduction of total volume w4iere calcite is changed into dolomite, and that 

 at a lower temperature than 300° F., under considerable pressure, nearly one-half of 

 the lime might be replaced, and a true double carbonate formed. We ought not, 

 however, to be satisfied by such speculations, and ought to persevere until artificial 

 pseudomorphs of dolomite after calcite have been prepared, in the same way as I have 

 prepared many pseudomorphs of great mineralogical and geological interest. The 

 partial conversion of the coral rock into dolomite is easily understood w^hen we see 

 that foraminifera, originally composed of calcite, are often only partially changed into 

 dolomite, and to some extent I'etain their original optical characters, but are more 

 and more changed in the coral rock from the deeper parts of the boring, the magnesia 

 being probably derived from the sea- water. 



