526 



BR. C. CALLAWAY ON THE GENESIS OF THE 



1. The Materials erom which the Schists were produced. 



Dio rites. 



I have been able to recognize at least four varieties of diorite in 

 the schists thus formed. 



Medium-black (No. 1). — This rock contains about equal pro- 

 portions of hornblende and felspar, and in the mass appears nearly- 

 black. The felspar, under the microscope, often presents a cloudy 

 appearance, and contains numerous clear microliths, both conditions 

 indicating alteration. When the alteration is only slight, the 

 twinning of plagioclase is visible, but this occurs only in a minority 

 of the crystals. 



Coarse-black (No. 2). — The hornblende is often in a greater pro- 

 portion than in No. 1. The crystals of both minerals are larger. 

 The felspars display similar alteration. 



Coarse-grey (No. 3). — The hornblende is usually about one fourth 

 of the mass. Most of the felspar is less changed, and shows the 

 striping of plagioclase. 



Medium-grey (No. 4). — The hornblende is abundant, but pale in 

 colour. The felspars display no twinning, are cloudy, with large 

 patches of opacite in the centre, and frequently contain microliths. 

 This variety will be but slightly referred to in the present paper. 



I am not prepared to say that none of these varieties ever graduate 

 into each other, but I have seen no evidence of a passage in any case. 

 No. 4 is the newest, for veins of it occur in No. 3 ; and No. 3 is 

 probably newer than Nos. 1 and. 2, since at North Malvern it con- 

 tains rounded and angular fragments of both. The occurrence of 

 these fragments would seem to indicate a consolidation previous 

 to the intrusion of the younger variety, and this is confirmed by 

 sections in the quarries at North Malvern. A mass of No. 2 was 

 seen to be penetrated by a variety of a lighter colour. The vein 

 passed between irregularly shaped joint-blocks, and contained de- 

 tached pieces of the darker kind. In another mass, a fine- 

 grained diorite was intrusive in a coarser variety, and in like 

 manner enclosed large blocks of the older rock. In this respect 

 the mode of intrusion is similar to that of the granite in diorite, 

 as described in my paper on the rocks of Galway. I have noticed, 

 however, that in Malvern, as well as the town of Galway, there was 

 a more thorough welding together of the two kinds of rock at 

 their junctions than was observable in the Pre-Silurian granite 

 and diorite of Connemara. 



Granite. 



I have no positive evidence of the existence of more than one 

 variety of original granite in the Malvern Hills. This is the well- 

 known binary compound of quartz and red orthoclase. 



This granite is younger than all the above-named diorites. By 

 its association with them it gives rise to some of the most interest- 

 ing phenomena of the region. 



