1865.] DUNCAN IMPKESSIONS OF SELENITE. 13 



1 inch to J incli in diameter, were empty and moist, and were usually, 

 but not invariably near a plant-bed. The impressions in the clay 

 were very distinct, sharp, and permanent, and the flatness of the crys- 

 talline planes was perfectly represented, as was also the sharpness of 

 the angles. The colour of the clay within and without the spaces was 

 at first the same, but exposure to the air determined the deposit of 

 sesquioxide of iron on the plane surfaces. Close to the exterior of 

 the spaces were numerous spots resembling Entomostracous remains, 

 but which were of a lighter colour than the clay, and which yielded 

 to analysis alumina, silica, and sulphide of iron*. When permitted 

 to dry, some of the hand-specimens showed much alteration near the 

 selenite impressions, from the replacement of the protoxide by the 

 peroxide of iron, but none elsewhere. Wo crystals or laminae of 

 selenite were found near the spaces. In one space there was a bril- 

 liant piece of carbon. 



The clays wei-e soft and not laminate ; they contained variable 

 quantities of sand, and graduated into a decided loam here and there. 

 The section proved that the natural drainage through the deposit 

 was in perfect operation. 



3. The mineral condition of the organic remains in the beds. — The 

 large Ostrece were imperfectly silicified and very hard. The Melanice 

 were generally very perfect, and were of all sizes : they were either 

 empty, very fragile, and the carbonate of lime in the shell was tender 

 in the extreme, or the shell was tough and its interior filled with 

 crystalline carbonate of lime ; or, in a few instances, an imperfect 

 siHcification had produced a cast. 



The Cyrence were of various sizes, and were either very fragile and 

 empty, or their interior was filled with dense carbonate of lime, the 

 shell being here and there deficient. 



The plant-remains were carbonized, and much of the surrounding 

 clay was stained black, but in some places fibres existed of their 

 usual colour. 



4. Discovery of the sjpaces in the London Clay. — Shortly after 

 these impressions were noticed in the Woolwich beds, several large 

 moulds of selenite were discovered in the London Clay of the 

 Tendring Hundred of Essex f. They were very numerous, and 

 were larger than those from the Woolwich beds. The clay in which 

 they were found was the usual dark blue -grey London Clay, and was 

 unfossiliferous. The impressions were perfect, very complicated from 

 the verticillate arrangement of the crystals, and the clay within the 

 spaces was either perfectly like that without, or after a time became 

 tinted with the sesquioxide of iron. Very small pieces of selenite 

 were abundant close to the impressions. 



5. Considerations respecting the deposits. — Selenite is very common 



* The loam when washed with distilled water yielded less than 1 per cent, of 

 soluble matters. They consisted of chloride of sodium, sulphate of magnesia, a 

 trace of organic matter, and less than 1 per cent of sulphate of lime. 



The finer particles of the loam took very long to settle, and the sediment con- 

 sisted of silica, silicate of alumina, protoxide of iron, and salts of lime, soda, and 

 magnesia. 



t Mr. James Cooke, C.E., F.G.S., obtained the specimens. 



