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deposited hexagonal plates, often in circular groups round the cavity. 

 The centre of the cavity is occupied by a granular opaque group of 

 crystals. The hexagonal plates are commonly of a brownish yellow, 

 and, if thicker, green ; but they are of extreme thinness. Some of 

 them exhibit dichroism by polarized light. 



The effects of pressure are detected on the mica near the cavities, 

 proving the force exerted. 



The plates of mica exhibit Newton's rings on certain circular 

 spaces, indicating the presence of air or gas between the laminae ; 

 and it is curious that wherever a cavity has projected liquid or gns, 

 it is situated on the circumference of one of these round spots. This 

 indicates that a gas has been projected between the laminae. 



The author then described a remarkable specimen, given him by 

 Dr Fleming, in which very thin hexagonal crystals of tourmaline, 

 in mica, were almost opaque. There are rectilineal cracks in them, 

 however ; and these, on looking at the sun through the crystals, ex- 

 hibit very beautiful optical phenomena. These fissures prove that 

 the crystals were soft after they had their present form. No cavity 

 appears in this specimen. 



This specimen, as well as others, contained the filaments and 

 sporules of Penicillium glaucum between the laminae. 



2. On Titanium in Mica. — Titanium occurs in mica, and often in 

 beautiful dendritic forms, mostly opaque, but when ^^Ve^^ of an inch 

 thick, transparent. When only a very thin film of mica is left over 

 them, the most beautiful colours are seen, due to the thin plate of 

 mica, and not to a vacuity. 



3. On the occurrence of Q,uartz in Mica. — The quartz occurs with 

 its axis of double refraction parallel to the laminae, but it never 

 occurred in regular crystals. 



4. On Titanium in Amethyst. — The titanium was found in fine 

 pyramidal crystals, coating the faces as with a powder, but covered 

 again with amethyst faces parallel to those within. The dust was 

 formed of spicular crystals crossing each other at angles of 60° and 30°. 



It would appear that the crystals of amethyst must have grown 

 in a solution which at times contained titanium, and at other times 

 did not. Hence the successive layers as the crystals grew. In one 

 case, the titanium formed the external surface in part of the faces 

 of the pyramid, as if the other part had lain in a liquid protected from 

 the deposit, and no more amethyst had afterwards been deposited. 



6. On Titanium in Topaz. — A number of imperfect crystals of 



