272 G L. Whittle — Ottrelite-bearing Phase of 'the 



different areas seem in the main to be accidental, although 

 locally they may be arranged parallel, as shown by a tendency 

 in the rocks to cleave into rude slabs, — a tendency augmented 

 by thin folia of sericite. A well-marked spherocrystalline 

 habit characterizes all the ottrelite areas ; in some this radiated 

 growth seems to be perfect. 



Microscopically the radiated structure is much more evident ; 

 composite and fan-shaped areas, penetrating one another irregu- 

 larly coexist with isolated prisms and beautiful spherocrystal- 

 line aggregates yielding imperfect crosses in polarized light. 

 Only sections cut parallel to the bundles of plates (basal sec- 

 tions) show well the radiated structure ; all other sections show 

 this character less and less, dependant upon the plane of the 

 section until it is transverse when the mineral appears prismatic. 

 The areas of the spherocrystals are not infrequently bounded 

 by overlapping six-sided plates of which three are usually free ; 

 the others are intergrown and confounded in the central por- 

 tion of the aggregate. 



The extreme mobility of the ottrelite-bearing solution is 

 indicated by the manner the ottrelite needles have insinuated 

 themselves into included feldspar grains along no visible lines 

 of Assuring. 



As in all described occurrences of this mineral an abundance 

 of inclusions exists. It is noticeable that while quartz and 

 occasionally feldspar are included, sericite, which is the prin- 

 cipal micaceous constituent of the rock, is seldom enclosed by 

 the growing ottrelite, but may be wholly or in part the nucleus 

 about which an aggregate formed. Such nuclei seem to have 

 governed the growth of the ottrelite. There is a tendency as 

 the mineral formed for the plates to orient themselves parallel 

 to the sericite nucleus so that when such a nucleus is surrounded 

 by basal ottrelite it is apt to be basal also. By far the most 

 abundant interpositions are a multitude of extremely minute 

 black to brown dots and aggregates. These with a No. 7 

 objective are resolved, in the main into rutile occurring in 

 knee-shaped and heart-shaped twins but generally in rounded 

 forms in which twinning is not distinguishable. In other 

 cases the highest objectives are incapable of individualiz- 

 ing the grains as is mentioned by Renard in his " Researches 

 Sur la Composition et La Structure Des Phyllades Ar- 

 dennes."* In some crystals the rutile is grouped in reticu- 

 lated lines conforming rather rudely to the planes of the two 

 principal cleavages ; but, as a rule grouped along irregular 

 lines that traverse the ottrelite and the groundmass alike, and 

 was arranged originally in structural lines, possibly depositional 



*Phyllade Ottrelitifere de Monthermee: Bulletin De Musee D'Hist. Nat. D 

 Belgique, 3, 1884, '85, p. 252. 



