142 MR J. D. FALCONER ON THE 



irregular. Usually a narrow band of augite, flecked and spotted with brown hornblende, 

 intervenes between the compact augite and the compact hornblende. The smaller 

 crystals may in this way be completely replaced by clove-brown hornblende, but 

 frequently the centre of such a secondary hornblende is occupied by a mass of chloritic 

 material representing the portion of the augite which succumbed to the other process of 

 decomposition already described. Frequently both processes may be observed in the 

 same crystal, a ring of undecomposed augite marking off the chloritic core from the 

 hornblendic margin. Plates of brown mica, also secondary, are occasionally found 

 associated with the hornblende, and both minerals, although more stable than the augite, 

 themselves apparently undergo a further decomposition. In the hornblende, the brown 

 changes to a green colour, which appears first in isolated spots and then spreads over 

 the whole crystal. At the same time a closer cleavage is developed, and the compact 

 hornblende becomes a mass of pale green or colourless fibrous actinolite. This green, 

 reedy hornblende, and also the biotite, change finally into scaly chlorite or into the 

 lamellar " delessite " described above. 



Although for descriptive purposes the hornblende has so far been considered 

 secondary, it ought not to be forgotten that much could be said in favour of its being 

 considered in primary intergrowth with the augite. The question has been repeatedly 

 discussed in penological publications, and it would seem that in most cases the same 

 evidence can be read both ways. Consequently one cannot but accept Rosenbusch's 

 conclusion that " definite discrimination between the two is only possible when the 

 hornblende occurs in its own form or in that of augite : if the external form fails, the 

 discrimination must always remain uncertain." # In these diabases I have been unable 

 as yet to find any definite primary outlines in the hornblende, and until these are found 

 it is most convenient, especially in view of the occasional occurrence of hornblende with 

 the outlines of augite, to consider the whole of secondary origin. 



Traces of the hornblendic decomposition may be found everywhere in the augite of 

 the dykes and sills, but it is in the columnar augites of the larger sills that this inter- 

 esting change reaches its greatest development. Curiously enough, it has progressed 

 farthest in those portions of the rocks which contain the largest quantity of mesostasis, 

 and even there it is to be noted that those portions of the augite crystals which abut 

 against the mesostasis are more completely amphibolised than those which are enclosed 

 in or surrounded by felspar. The plagioclase seems in some way to have preserved the 

 augite from decomposition or at least hindered the process, t 



In various portions of the dykes and sills a rhombic pyroxene is an abundant 

 accessory. Its distribution, however, is exceedingly sporadic, and in any one sample its 

 presence can never be guaranteed. It appears to be most commonly associated with 

 the ophitic or sub-ophitic augite, and builds either large irregular plates or short stout 

 columns idiomorphic in the prism zone. As a rule, it has crystallised out before the 



* Mikroskopische Physiographic, etc., vol. ii. p. 1108. 



l Of. Holland, Quart. Journ. Oeol. Soc, vol. liii., 1897, p. 405. 



