G. W. Tyrrell — Intriisiom of Kilsyth- Croy District. 307 



There is no sharply defined boundary with the augite, and irregular 

 flecks of hornblende occur within the unaltered augite. In this 

 change iron has been thrown out of combination, and reappears as 

 secondary magnetite associated with the hornblende. 



Green chloritic or ' viriditic ' minerals are the product of another 

 type of alteration. These are generally fibrous, with the fibres 

 arranged in curved bundles or sheaves. They are highly pleochroic, 

 the extremes of colour being straw- j'ellow and grass-green, or in 

 a highly ferriferous variety from reddish brown to a dark green. This 

 m;iterial certainly includes several varieties of chlorite, and is frequently 

 intergrown with highly pleochroic brown biotite. 



The basal striation occurring in the augites of the coarser rocks 

 affords a starting-point for a distinct type of alteration. This occurs 

 especially in the interior of a crystal, and consists of an accumulation 

 of dusky granules of calcite arranged along the planes of the basal 

 striation. 



The final stage of both the hornblendic and chloritic types of altera- 

 tion is the production of a pale-green serpentine uniformly sprinkled 

 with specks of separated magnetite. The serpentine is slightly 

 pleochroic, and is sometimes laminated. It is then frequently inter- 

 grown with brown hornblende. The bright polarization colours of 

 augite are still retained in isolated areas, but the rest of the crystal 

 possesses the aggregate polarization of serpentine. This type of 

 alteration is well seen in the coarse rocks from Dumback, Fankerton, 

 and Milngavie. In the latter rock the augite appears to pass directly 

 into serpentine without intermediate stages. The serpentine makes 

 its appearance along the cleavage and other cracks, leaving isolated 

 areas of unaltered augite. Amphibolization is very rare in the 

 Milngavie specimens. 



In his study of similar rocks from the Bathgate Hills, Dr. Falconer ' 

 has arrived at the conclusion that amphibolization or chloritization of 

 the augite occurs according as the alteration starts from the exterior 

 or the interior of a crystal. There is nothing in the rocks under 

 consideration to support this view. Both types of alteration appear to 

 start indifferently from the exterior or interior of the crystal. 



There is evidence for some local selective attack by weathering 

 agents in the fact that alteration rarely occurs to the same extent in 

 both felspar and augite. Where one is much altered the other is 

 nearly or quite fresh, and vice versa. Thus, in a specimen from 

 Barrwood Quarry, Kilsyth, the felspars are completely kaolinized, 

 whereas, save for a slight peripheral amphibolization, the augite is 

 quite fresh ^ ; but in a rock from Nethercroy, about half a mile distant 

 from the above locality, the felspars are quite fresh, whereas the 

 augite is completely altered to a fibrous brown hornblende or to 

 a greenish-brown serpentine. 



An orthorhombic pyroxene occurs with the augite in some specimens, 

 but is very sporadic in its occurrence. It occurs in prismatic, much 

 cross-fractured, faintly tinted crystals, which in suitable sections 



1 Falconer, T.R.S.E., vol. xlv, pt. i, p. 141. 



2 Cf. Teall on the Corstorphine rock, Brit. Petrog., 1888, p. 190. 



