516 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOR AND C. A. SUSSMILCH. 



absorbed from the shale. As regards the felspar and olivine, 

 the continuous change becomes apparent at a distance of 

 only 30 cm. from the contact (compare columns II and I), 

 but in this case it may be illusory, and due to the great 

 horizontal distance separating the specimens, or to some 

 other cause, and may not be connected with the general 

 change shown by succeeding specimens. Felspar, which 

 at the periphery composes about 38 per cent, of the rock, 

 forms 45 per cent, at 4J m. from the junction, rapidly 

 increasing to 50 per cent, at 5 metres from the contact, 

 and reaching 56 per cent, a metre or so further down. 



Though in this section we are treating properly only of 

 massive varieties, it is necessary to say a few words here 

 regarding the aplitic veins which are found traversing the 

 mass, and which will be described in detail later. All the 

 larger of these veins, or more correctly sheets, have a 

 horizontal disposition. The uppermost is met in both 

 quarries at about 5 m. (16 feet) below the junction, that is 

 just below the level of specimen P. At depths respectively 

 of about 13 and 17 metres are seen in the Reservoir Quarry 

 two much thicker segregation veins. An analysis of a 

 specimen of the coarse aplite from one of these was made, 

 and its mineral composition, which probably represents 

 fairly accurately the composition of all the aplitic veins, 

 has been calculated from this, and is added to the above 

 table (column VI). Evidently the aplitic rock is the climax 

 of the progressive changes of composition just described ; 

 the felspar has risen to 78 per cent., orthoclase which has 

 risen from 1 to 5 per cent, now counting 18 per cent.; olivine 

 and biotite have disappeared altogether ; and augite has 

 continued its slow but continuous diminution. 



All the changes so far described take place within five 

 metres of the contact. Continuing downwards, the rock 

 remains of a felspathic character, having probably a com- 



