Mr. W. Murton Holmes on some Forms of Silica. 213 
trench in Chepstow Rise, a road about 220 yards west of this 
point. The occurrence of selenite in the bed D also points to 
its being London Clay. 
The pebble bed was found in drain trenches in Park Hill Rise 
to be 2 ft. in thickness at a point 126 yards north-west of 
Chepstow Road, and 4 ft. thick about 88 yards south-east of 
that road. 
In a sewer trench in Addiscombe Road, about 200 yards west 
of the point where it is crossed by the Woodside and Selsdon 
Road Railway, a yellow sand with thin partings of clay was met 
with, similar to the bed C in Park Hill Rise; this was capped 
with a thin layer of gravel. Just east of the same railway 
bridge, a sand-pit has been opened in 1896, which exposes a 
fine section 15 ft. or more thick of the same yellow sand with clay 
partings. In the top soil above this sand are numerous rounded 
pebbles, obviously derived from the Oldhaven pebble bed, which 
would seem formerly to have been present above the yellow sand 
in this locality, as it is in Park Hill Rise. On the other hand, 
in the railway cutting between the two tunnels the pebble bed, 
containing shelly conglomerate exactly similar to that met with 
in Park Hill Rise, is situated below the yellow sand, as shown 
in Mr. Klaassen’s section. I can only account for the different 
position of the pebble bed in the different places by supposing 
that, notwithstanding the exact similarity of lithological char- 
acters and fossil contents, there are really two distinct pebble 
beds, one above and one below the yellow sand, and marking 
different episodes in the deposition of the Oldhaven beds in this 
locality. 
In the laying of the sewer in Chichester Road the shelly 
sand G was found to extend only a short distance westward 
from the junction with Park Hill Rise; the remainder of the 
trench as far as Chepstow Rise being in mottled clay with shelly 
and sandy layers, similar to the bed F in the Park Hill Rise 
section. No sign of the fault was seen; probably it lies further 
to the north. 
130.—Somu Forms or Sixica. 
By W. Murton Hotmzs. 
(Read April 28th, 1896.) 
Smica, the oxide of a non-metallic element called silicon, 
is the most abundant and widely distributed substance in the 
mineral kingdom, and either alone or in combination with other 
aeenmoce forms more than one-half of the known crust of the 
earth. 
