146 MR J. D. FALCONER ON THE 



(a) The Aphanites. 



The mineral composition of the aphanites is similar to that of the diabases. The 

 felspars are rarely zoned, and the earlier formed basic varieties are usually more 

 idiomorphic than the later acid types which assume very generally a ragged, elongated 

 form. The augite may occur in the granulitic condition between or partially enclosed 

 in the felspars, or it may build small ophitic plates and act as matrix. More rarely it 

 develops a long prismatic branching form. (See PI. III. fig. 1.) Occasionally in a 

 section earlier formed glomero-porphyritic groups of augite and felspar may be detected 

 by their compact structure. Paramorphism to hornblende is rare ; chloritisation is more 

 common. Magnetite occurs in skeletal networks. Pyrites is usually abundant, frequently 

 in porous cubes. Olivine is occasionally found, but never bronzite. A little undifferenti- 

 ated groundmass is sometimes present, also a little quartz, and very rarely a patch of 

 granophyric material. The structure is minutely granular or ophitic according to the 

 habit of the augite. Vacuoles of irregular shape, and filled with calcite, chlorite, and 

 quartz, are common. (See PI. III. fig. 2.) Many of these are probably not to be 

 considered secondary in the ordinary sense of the word. It is frequently evident that 

 the quartz has been formed by a molecular replacement of various minerals, especially 

 felspar, and sometimes the ghosts of lath-shaped felspar can be seen outlined in the 

 quartz by means of their original inclusions which have been preserved within the 

 pseudomorph. The ocelli on the other hand represent primary steam -cavities, partially 

 or wholly occupied by residual material leached out of the intersertal spaces during the 

 final stages of consolidation.* (See PI. III. fig. 3.) In nature they are essentially 

 felspathic, being filled as a rule by an irregularly felted mass of slender, curving rods of 

 felspar, acicular crystals of purple augite, usually undergoing decomposition, a little 

 apatite and quartz, and frequently much pyrites in open porous crystals and irregular 

 growths. One or more vacuoles, representing primary miarolitic cavities and filled with 

 calcite, quartz, or chlorite, are usually present within the ocellus. 



(b) The Diabase Porphyrites. 



These occur at the contact, and are minutely porphyritic with olivine, augite, 

 plagioclase, and glomero-porphyritic groups of augite and felspar. (See PI. III. fig. 5.) 

 The olivines are usually much corroded and decomposed. Much xenocrystic quartz 

 with augite mantles is usually present, and probably derived from the adjoining sedi- 

 mentary rocks. (See PL III. fig. 6.) Pyrites is frequently abundant in irregular 

 granular masses or porous cubes. The groundmass is exceedingly fine-grained, and 

 consists of microlites of felspar, granules of augite, much magnetite dust, and probably 

 also a small quantity of undifferentiated material. 



* Teall, Geol. Mag.,>1889, p. 481 ; Flett, Trans. Roy. Soc. Edin., vol. xxxix., 1900, p. 865. 



