1885.] 41 [Genth. 



Fritz's Island. Sinking Spring. 



al a 2 61 6 2 cl c2 



Water = 30.92 — 32.52 — 29.70 — 29.47 — 29.91 — 31.05 



Carbon dioxide = 2.42 — 



Silica and alumina = — 0.46 — 



Ferric oxide = 0.82— 0.44— 0.30— 0.04— 0.75— 1.24 



Manganous oxide = 0.63 — 4.04 — 4.66 — 



Magnesium oxide = 67.64 — 66.78 — 65.38 — 64.30 — 66.62 — 66.19 

 Calcium oxide = 0.11 — 1.68 



100.01 — 99.74 — 99.42 — 98.93 — 99.81 — 100.16 



The 2.42 per cent of carbon dioxide in analysis c 1 indicate the presence 

 of 0.20 per cent of calcium carbonate, and about 4.45 per cent of mag- 

 nesium carbonate or about 6 per cent of hydromagnesite, resulting from 

 a conversion of a small quantity of brucite into these minerals — there are 

 still, however, over 90 per cent of unaltered brucite present. 



Deweylite, Aragonite, Calcite. — In the magnesian limestone occur these 

 three minerals, more or less mixed together and associated with brucite. 



The deweylite is white, yellowish-white or brownish, amorphous, some- 

 times in rounded grains or in stalactites or botryoidal forms, in thin plate- 

 like masses or slabs occasionally over one inch in thickness, or in irregular 

 coatings. These slabs are often arranged in layers of white or brownish 

 deweylite of greater or less purity, often intimately mixed with aragonite, 

 which sometimes separates in the form of radiating columnar masses, 

 some of the individuals being over 50 mm in length. The layers often sepa- 

 rate very easily and the surfaces of such planes of separation are covered 

 with small brilliant crystals of aragonite. 



Calcite is also present, both in small and insignificant crystals and in 

 coarse crystalline masses. 



This deweylite has been analyzed by E. F. Smith (1. c), also by my 

 assistant, Mr. H. F. Keller, who found a pure yellowish fragment of 

 waxy lustre to contain : 



Silicic oxide = 39.32 



Ferrous oxide = 0.51 



Calcium oxide = trace 



Magnesium oxide = 41.14 



Water = 18.41 



99.38 



Neither the aragonite nor calcite have been analyzed. 



Pseudomorph, of deweylite after aragonite. — The needle-shaped crystals of 

 aragonite and the radiating masses undergo a change and are gradually 

 altered into brownish-yellow deweylite. 



It begins with a very thin coating of colorless and brownish-yellow 

 deweylite upon the aragonite, .which gradually becomes thicker and final- 

 ly changes the entire aragonite into pure deweylite. 



PllOC. AMEK. PHILOS. SOC. XXIII. 121. F. PRINTED OCTOBER 12, 1885. 



