90 MR. R. H. WORTH OX THE [vol. lxXV, 



quarry the dyke at LXXVI, S.E. 58 is met. This has been worked 

 through ; it shows, however, not onty in the present face of the 

 quarry, but also in a spur of rock left on the south-west. Its thick- 

 ness is almost 95 feet. LXXVI, S.E. 59 is another exposure, 

 also showing on two faces of the quarry. It is bordered, especially 

 on the northern margin at 59, by parallel reins. Next there is 

 a natural exposure which crops out with many large boulders at 

 LXXVI, S.E. 75, on the hill south of the L. & S.W. Railway 

 quarry. At 15 it sends off a narrower branch from its southern 

 margin, making an angle of about 24° with the principal belt of 

 rock. The broader belt continues, passing under the viaduct, and 

 reaches the bank of the West Okement at 71. The river-bed is 

 covered with shingle and boulders, and the western bank is over- 

 lain by quarry- waste and the railway embankment: hence it cannot 

 be proved that a sill extending from LXXVI, S.E. 70 to 68, is 

 continuous with the 75-71 exposure, but the continuity is ahnost 

 certain. This sill varies between 12 and 20 feet in thickness. 

 LXXVI, S.E. 84-67-66-80-82 is a dyke of rock which has been 

 classed by McMahon as a mica-diorite. The mica is wholly 

 secondary, and specimens can be found almost free from this 

 mineral ; the structure is doleritic, and I prefer to class it as a 

 clolerite — it has distinct affinities with the rock at 58 in the 

 L. & S.W. Railway quarry. LXXVI, S.E. 77-76, is the southern 

 margin of a band of ' dark igneous ' rock. Other exposures occur 

 along South Down, as at LXXVI, S.E. 22, a little south of this, 

 from which point a band extends eastwards to LXXVI, S.E. 25, 

 where it broadens into and interrupts the main dyke of Meldon 

 .aplite, which, however, persists as veins and minor dykes. Here 

 also the ' dark igneous ' rock comes into contact with the f elsite with 

 micropegmatite. If surface-indications can be trusted, the ' dark 

 igneous' rock ranges hence, eastwards, side by side with the aplite, 

 until it dies out in the aplite-quarry on the western bank of the 

 Redaven. Although the ' dark igneous ' rock dies out in this quarry, 

 it is obvious from surface-indications that it occurs again on the 

 course of the aplite-dyke north-east of LXXVI, S.E. 17, since 

 blocks of 'dark igneous' rock penetrated by veins of aplite are 

 found. Following the Redaven upwards from the Meldon Valleys 

 Company's quarries we reach LXXVI, S.E. 39, where there is an 

 important exposure which can be well seen in the river-bed. 

 The last-known ' dark igneous ' rock in this direction is found as 

 surface-blocks at LXXVI, S.E. 7, on the hill above the Redaven 

 mine ; as this lies in the direction of the course of a small dyke of 

 Meldon aplite, seen in the bed of the stream at LXXVI, S.E. 27, 

 it is not surprising to find the aplite as veins in the ' dark 

 igneous ' rock. 



Probably there are other veins, dykes, or sills of the ' dark 

 igneous ' rock which are hidden from observation ; prior to the 

 quarrying no one would have ventured to suggest that three 

 separate outcrops of this rock occurred on the site of the London 

 <fc South- Western Railway quarry. (The rock is prominent on 



