part 21 (iEOLOGY OF TI1K Mi:j,l)()N FALLETS. lOO 



Apatite is invariably l>lu.\ reaching a deep indigo where it is 

 freely developed on joint -faces. It is doubtful whether it is 

 entirely absent front any section of the aplite; in some it assumes 

 iniportanee as a rock-£ornaing mineral. 



The azure colour of the Meldon aplite where it has been chilled 

 is entirely due to very line granular apatite, as mentioned ahove. 

 From this dust-like form it increases near some joint-faces to dimen- 

 sions of over 2 mm.; slide Q. Ill, 82, from a vein in the aplite, 

 shows apatite prisms 3mm. in length, with good hexagonal cross- 

 sections -(J mm. in diameter. 



The larger crystals have distinct pleochroism in shades of blue, 

 the smaller grains in section are sometimes colourless. Fluorspar 

 occurs in cracks in the apatite-crystals. The larger apatites fre- 

 quently show strings of cavities such as occur in quartz : some of 

 these contain fluid and a bubble, but the greater number appear to 

 have been filled with purple fluorspar. Especially in sections from 

 near joint-faces, a close association between tourmaline and apatite 

 is to be noted, as also many instances of adjacent areas of the 

 two minerals extinguishing simultaneously (Q. I, 43). 



Veins rich in apatite traverse some parts of the Meldon aplite, 

 in which their distinctive colour renders them prominent. 



Pleochroic halos occur around small opaque inclusions ; these 

 halos are so faint that their measurement is difficult, but a radius 

 of about •015 mm. seems a good approximation. 



Fluorspar. — This is more essentially a mineral of the joint- 

 faces and some of the coarser veins. In colour it is mainly purple, 

 but occasionally green. Under the microscope it is seen that the 

 colour is irregularly splashed throughout the crystals, some of 

 which are zoned, either in purple or more rarely in blue — both 

 colours may occur in the same crystal. 



One joint-face in Q. I, east of the Redaven, is lined with a 

 detachable film a little less than 1 mm. thick. Sections show 

 this film to consist of fluorspar and quartz : the latter much 

 clouded in parts, but sometimes in clear sharp crystals ; the former 

 colourless, except for purple and blue zones. Fluorspar occurs as 

 inclusions in mica and in apatite, and associated with axinite. 

 It is present in varying quantity in different parts of the main 

 dyke, and is as prevalent at Sourton Tors as at any other locality. 

 Occasionally a vein rich in fluorspar traverses the main dyke. In 

 the minor dykes the mineral is extremely rare. 



Montmorillonite was first reported by Mr. H. J. Lowe (7). 

 Since his discovery the development of the quarry has yielded 

 some very fine specimens. It is especially prevalent near the 

 Redaven, and the best examples all come from the coarser 

 veins. Of a beautiful rose-colour, it has a waxy lustre, and can be 

 scratched by the thumb-nail. It forms an apparent matrix, in 

 which are set crystals of white felspar, quartz, mica, and green 

 tourmaline. It would appear to be an alteration-product of some 



