GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 529 



thin veins, but from point to point are everywhere altern- 

 ately swelling and thinning out again in an irregular 

 manner. In the thinnest parts they are sometimes reduced 

 to as little as 15 cm. in thickness, where largest they 

 reach a width of 120 cm. Their average thickness is per- 

 haps about 60 cm. In places, however, the veins swell up 

 into oval or irregular shaped masses, from which one or 

 more subsidiary veins often branch off. The largest of 

 these (Fig. 4) is shown on Plate XXXV., near the letter H, 

 and is described and figured in the next section. It is 

 nearly 3 m. thick, and more than twice as long. 



Fig. 4. — Sketch of part of one of the large Segregation Veins. 



In the Emu Quarry segregation veins are common in 

 much the same degree as in the upper levels of rock 

 exposed in the Reservoir Quarry, and the upper of the two 

 great veins is exposed in the deeper part of the quarry at 

 its north end, so there is evidence that they have a wide 

 extension. Exposures in other parts of the mass {e.g. 

 Booth's Quarry) show that segregation veins, chiefly aplitic, 

 are widespread in the mass at a level immediately below 

 the most compact pallio-essexite. Neither pegmatitic nor 

 aplitic veins have been found penetrating and intruding 

 the shales anywhere at Prospect. 



18. Megascopic Characters and Relations of the Rocks 

 Composing the Veins. 



The rocks filling the segregation veins vary both in 

 composition and texture. As already stated they may be 



