276 SCIENCE PROGRESS. 



same certainty about the original minerals from which 

 pseudomorphs were derived. In far too many instances 

 the wildest and most diverse views have been held about 

 the initial composition of the crystals, sometimes owing to 

 the fact that very similar, though not the same, forms may 

 be characteristic of several different minerals. 



Pseudomorphs of lead carbonate having precisely the 

 common form of barium sulphate have been found at 

 several localities, and have generally been called pseudo- 

 morphs of Cerussite after Barytes ; it is obviously far more 

 reasonable to suppose that they were derived not from 

 barium, but from lead sulphate which is isomorphous with 

 Barytes, and therefore has almost the same form, and that 

 the pseudomorphs are really after Anglesite. Again, the 

 Serpentine found at Brewster in New York, which pre- 

 sents a most perfect cubic cleavage, was referred to various 

 cubic minerals which bear absolutely no relation to Ser- 

 pentine, until Tschermak made the extremely probable 

 suggestion that they are due to the alteration of Periclase, 

 the cubic oxide of magnesium. That Periclase is else- 

 where only found in minute crystals is no argument against 

 such an explanation. A similar objection was raised to the 

 idea that the altered tetrahedral crystals found on the River 

 Achtaragda in Siberia, and known as Aehtaragdite, might 

 have been derived from the rare mineral Helvite which they 

 resemble in form ; yet crystals of Helvite quite as large as 

 any of the Aehtaragdite crystals were subsequently found at 

 two or more localities. 



Again, carbonate of lime is sometimes found in acute 

 pyramidal crystals, under such conditions that it might 

 reasonably be supposed pseudomorphous after Celestine ; 

 until a recent date this seemed an unlikely explanation, for 

 Celestine was not known to occur in such forms ; yet there 

 is clearly no reason why the mineral should not grow in 

 pyramidal crystals, provided that they have angles character- 

 istic of Celestine, and it has recently been found at several 

 localities in acute forms very similar to those of the pseudo- 

 morphs. 



These are only a few examples out of many which might 



