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650 PRESIDENT'S ADDRESS. 



the Mount Gambler limestone ; but however far apart the localities 

 where the clei^osits were represented they never varied in character. 

 The formation is very extensively spread over thousands of 

 square miles of country, from the Eiver Murray to Gippsland, 

 but it does not always crop out on the surface. It is covered by 

 a formation in which all the shells are well preserved, and where 

 such things as casts are unknown. I do not propose to dwell on 

 the strong contrasts these deposits present, but merely draw 

 attention to the fact that in the lower Br3'Ozoan limestones the 

 majority of the shells and corals are always found as casts, and 

 a few have the shelly matter unchanged. I was astonished at 

 finding that the rule held good in the same formation in as remote 

 a locality as the Middle Island of New Zealand (Oamaru). It 

 appears that the same phenomenon is occasionally seen in Europe 

 and there the same shells fPectens, Brachiopoda, &c.) resist the 

 solvent, and are found entire. I must refer my readers to the 

 address of Mr. Sorby for a full account of this most singular case 

 of metamorphism, and for the details of the ingenious and 

 and brilliant researches >vhich have led to its explanation. The 

 results are briefly these : Mr. Sorby has found, b}^ careful analj^sis 

 that much of the carbonate of lime in Shells, Corals, Bryozoa, 

 Echini, &G., is present in a mineral or crystalline state, and not 

 merely organically combined. There are, I need hardly sa}^ two 

 well defined forms for the crystals of carbonate of lime — one 

 calcite, and another arragonite. It is in the latter form that the 

 mineral is found for the most j)art in shells, &c., the exceptions 

 being few. But arragonite is a very unstable form of the 

 combination of carbonic acid and lime, and thus it is easily 

 decomposed, and dissolves away or changes into calcite. The 

 latter mineral is a very stable form, and though the pseudo- 

 morphs of calcite in arragonite casts are numerous, the contrary 

 never happens, that is to say, arragonite is never formed at the 

 expense of calcite crj'stals. Furthermore, Mr. Sorby has found 

 that Shells, Corals, &c., whicli invariably occur as casts, are those 



