386 



MR. E. WETHERED ON THE DEVONIAN 



the Carboniferous Limestone of Clifton, near Bristol, well-rounded 

 grains of detrital quartz were found/ but in the case of the 

 Devonian Limestone residues which I have examined, no such 

 detrital graias have been met with. 



As to the zircons, and what little tourmaline and ordinary rutile 

 there is in the residues, these may be of direct detrital origin, but 

 the sharply-defined angles of the crystals constitute an argument 

 against this presumption, and I am rather disposed to regard these 

 crystals as having been liberated by the decomposition of minerals 

 in which they were originally included. 



Considerable interest centres around the very minute crystals 

 which I have referred to as ' microlitic needles.' I have adopted 

 this term for convenience, and because of the resemblance of these 

 minute objects to the ' clay-slate needles ' of rutile. They are noty 

 however, always straight (PI. IX. fig. 4). Indeed, it is most difficult 

 to determine the true form of these crystals. They occur in every 

 one of the residues that I have examined, and, as I have before 

 pointed out, are so numerous as to be an important feature. - 



The needles appear in the very fine residue, which is usually a 

 siliceous and micaceous ' paste ' "'* ; also as inclusions in siliceous 

 and micaceous flakes (PI. IX. fig. 4), and in such numbers as to 

 import a dark hue to these objects, which seem to be remnants of 

 decomposed minerals. There is little doubt that the paste is formed 

 by the breaking-up of the siliceous and micaceous flakes, and this 

 explains the presence of the needles. 



I have said that the microlitic needles resemble 'clay-slate 

 needles' of rutile in some respects. To determine accurately the 

 optical properties of objects so minute is obviously a most difficult 

 matter. It is, of course, possible that they may be identical with 

 the clay-slate needles of rutile, and they certainly correspond 

 closely with those figured by Eosenbusch.^ I have not, however, 

 observed twin crystallization, so characteristic of that mineral. 



The fact that the microlitic needles occur so frequently in 

 siliceous fragments containing liquid inclusions does nor necessarily 

 imply any connexion between the liquids and the needles, yet it is 

 impossible to escape the thoxight that possibly there may be a con- 

 nexion between the two. 



The discovery of micro-crystals of quartz in limestone is not 

 new. I have referred to them as occurring in the Carboniferous 

 Limestone of Clifton ' and in the Inferior Oolite of the Cotteswold 

 Hills. ^ Prof. Sollas ^ also notes the occurrence of micro-crystals of 

 quartz in Carboniferous Limestone from Caldon Low, Derbyshire, 

 and in a footnote he states that " similar bi;t much larger 



1 Quart. Journ. Geol. Soc. vol. xliv. (1888) pi. viii. fig. 1 . 

 - To make them out a good light is required ; I find lamp illumination the 

 most efieetive. 



^ A term which I borrow from Mr. Hutehings, Geol. Mug. for 1890, p. 269, 

 ^ Microseop. Physiogr.' (transl. Iddings), pi. xv. fig. 4. 



5 Quart. Journ. Geol. Soc. vol. xliv. (1888) p. 191. 



6 Ibid. vol. xlvii. (1891) pi. XX. fig. 6. 



■^ Ann. Mag. Nat. Hist. ser. 5, vol. ii. (1878) p. 361. 



