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Transactions. 



are bedding-planes, possibly in some cases distorted by the chemical re- 

 arrangement that has taken place. 



The flint-beds are in most outcrops much shattered, and it is difficult to 

 collect hand specimens free from flaws, although more compact specimens 

 may be obtained from the gravels of the streams cutting through the 

 series. In general the boundary between the light-coloured exterior and the 

 inner dark flint is sharp, but there is no surface of easy separation between 

 the two, and flints of the Chalk type preserving their original surface cannot 

 be obtained either naturally or artificially. The amount of the light- 

 coloured exterior varies considerably from place to place, and there is also 

 considerable variation in its composition. It may consist of dark flint 

 containing more and more rhombohedra of carbonate as the exterior is 

 approached, until finally a rock is obtained which consists only of carbonate, 

 and resembles marble. Such a gradation from dark flint has been observed 

 in boulders obtained from the lire Eiver, and in this case there is no sharp 

 boundary between the inner and outer layers. In the usual case there is a 

 sudden change from a flint practically without carbonate crystals to one 

 containing them abundantly, the outermost layer again consistirig almost 

 entirely of carbonate crystals without any flint matrix. Occasionally 

 thin lenticules of flint occur within beds of limestone, and there may or may 

 not be an intermediate rock with crvstals of carbonate set in either a flint 



Fig. 1. — -Flint with relatively few euhedral crystals of dolomite, Ure River gravels. 



Magnified 15 diameters. 

 Pig. 2. — -Dolomite rock with a little flint in the interstices of the crystals, Mead Gorge. 



Magnified 15 diameters. 



or a partly calcareous matrix. Text fig. 1 shows the appearance in micro- 

 scopic section of a flint with relatively few crystals of the carbonate, which 

 are euhedral towards the flint, but anhedral towards one another. Text 

 fig. 2 shows the carbonate rock with only a little flint in the interstices of 

 the crystals. Where larger crystals of carbonate occur in a fine-grained 

 calcareous matrix they are less regularly euhedral than when they occur 

 in a flint matrix. 



The size of the carbonate crystals is approximately constant in any one 

 specimen, but varies considerably in difl'erent specimens, a common size 

 being a little over \ mm. in the greatest diameter. One exceptional speci- 

 men, obtained from the gravels of the Isolated Hill Creek, a tributary of the 

 Ure River, contains rhombs with a largest diameter of as much as 11 mm. 



