528 



DR. C. CALLAWAY ON THE GENESIS OE THE 



with granite-veins ; but at intervals, and especially at the southern 

 end, a banded structure has been produced. The northern part of 

 Swinyard's Hill is composed of massive granite, while at the 

 southern end is a flaggy gneiss, which has been produced from the 

 granite by pressure. Other examples of the localization of pressure 

 occur in the hills further south. 



3. The Products oe the Metamorphism. 

 A. Simple Schists, or those formed from one kind of rock. 



Hornblende-gneiss, formed from Diorite. — In this case the pres- 

 sure has rearranged the constituent minerals, so that the horn- 

 blende and felspar lie in irregular folia. There are many grada- 

 tions between the ordinary diorite and a gneiss in which there has 

 been some reconstitution of the minerals, the formation of quartz 

 being the most conspicuous result. The coarse-black diorite, at 

 North Malvern, is one variety which has been modified into gneiss. 

 For about two thirds of the breadth, the intrusion of the vein was 

 clearly seen ; then, for a few inches, the two kinds of diorite were 

 confusedly mingled. The remainder of the breadth, consisting only 

 of the coarser diorite, was rudely foliated. 



Mica-gneiss, formed from Granite. — Near the southern end of 

 Swinyard's Hill, on the crest of the ridge, is a very interesting- 

 case of the formation of gneiss by crushing. A narrow band, 

 striking across the axis, has the appearance of the ordinary binary 

 granite ; but a laminated structure is very apparent even in the 

 field. In immediate contact, forming part of the same mass, the 

 rock is flaggy, and seams of mica appear. Then comes a break, but 

 flaggy schists of the same general type appear in force a few 

 hundred yards to the north. A description of microscopic slides 

 will bring out the transition indicated. 



No. 281. This was taken as a typical specimen of the granite 

 which is seen in mass at the northern part of the hill. It is the 

 ordinary compound of orthoclase and quartz, with a little mica. 

 Most of the felspar is suffused with a brownish tinge, probably iron- 

 oxide, and presents a cloudy appearance. Many of the crystals 

 also contain patches or microliths of clear mica, polarizing in 

 brilliant colours. The felspar has therefore undergone partial 

 decomposition. That this granite contains iron would appear from 

 the analysis of the Rev. J. H. Timins*. He states that the 

 " quartzo-felspathic " veins, which are almost certainly the granite 

 I am discussing, furnished in three analyses iron-oxide varying 

 between '92 and 1-52 per cent. Both quartz and felspar are some- 

 what cracked, and iron-oxide is deposited in the cracks. 



Nos. 282-284. — From the locality at the southern end of the hill. 



No. 282. Prom a part of the laminated granite which appears 

 more granitoid than the rest. The rock is excessively cracked and 

 crushed, the cracks, which run in all directions, but predominantly 



* Quart. Journ. Geol. Soc. vol. xxiii. p. 362. 



