Major- General MacMahon — Rutile in Fireclays. 259 



destruction of others. Some material of this kind representing 

 Fritschia, Hylerpeton, etc., is still only in process of development, 

 and may perhaps yet enable their skeletons to be reproduced more 

 perfectly than heretofore. 



Reference to Plate VIII. 



(1) Skull and Maxillae ; {la) Sternal and Scapular bones ; (2) Mandible; 



(3) Humerus, llibs and Vertebrae ; (4) Hind Limb ; (5) Pelvis ; 



(6) Caudal Vertebrte. 



V. — Note on the Alleged Genesis or Rutile in Fireclays. 

 By Major-General C. A. MacMahon, F.G.S. 



ME. W. M. HUTCHINGS' " Notes on the Fireclays of the Coal- 

 measures," published in the April Number of the Geol. Mag., 

 contains many interesting facts and suggestions on which I should 

 like to offer a few remarks. 



The conclusion at which the author appears to have arrived (see 

 para. 2, p. 168, read with the two bottom paras, of that page) is 

 that the formation of the " rutiliferous mica," and its contained rutile, 

 is due to dynarao-metamorphism. 



This is rather a startling conclusion and it is one which seems 

 to rest on slender evidence. Even if it could be proved that the 

 rutiliferous mica and the free rutile-needles are both of secondary 

 origin, no evidence has been adduced to show that their genesis 

 is due to dynamic and not to ordinary aqueous agencies. Evidence 

 on this point is the more desirable as one does not usually associate 

 beds of unindurated clay with the display of dynamic energy. Do 

 fossils of these Coal-measures exhibit pressure deformation ? 



Even in cases where sedimentary rocks have evidently suffered 

 deformation from earth-movements, it would not be safe to assume, 

 without proof, that the secondary minerals found in them owe their 

 birth to dynamic causes. 



With respect to the Seaton beds there are two distinct issues to 

 be proved. Are the rutile-needles and the yellow mica of secondary 

 or of clastic origin ? And if secondary, to what process do they owe 

 their birth? 



Regarding the first issue I remark that some of the author's facts 

 seem quite consistent with the view that the rutiliferous mica, and 

 the free rutile-nodules, were transported to the spot along with the 

 other constituents of the clay. The finest washings " A, B, C," con- 

 tain " far away the largest portion of the rutile-needles ; " and 

 many of the quartz and felspar grains carry "more or less a skin 

 of rutiliferous 'paste' in spite of all the washing and agitation. 

 I think it is," Mr. Hutchings adds, " really corroded on to the 

 substance of the grains in many cases." These facts do not give 

 much support to the theory that the mica and its endogenous rutile 

 " is a new formation posterior to sedimentation." 



Another fact mentioned by the author seems to point to the same 

 conclusion : he tells us that the flakes of rutiliferous mica, with few 

 exceptions, are seen by their extinctions to be " aggregates of more 

 or less numerous smaller flakes overlapping one another, and with 



