204 YORKSHIRE NATURALISTS’ UNION IN UPPER SWALEDALE. 
At Oxnop (or Ousnop) Beck Head the party spent some time in 
examining the road-side sections of the Main Chert and Black Beds. 
These were seen to be silicious (not arenaceous) limestones, in which 
silica in a colloid form had been an original constituent of the rock. 
It is chiefly of organic origin, consisting largely of sponge specules. 
Locally it passes into beds of nearly pure chert, and graduates 
horizontaily, in other directions, into limestones of the ordinary type, 
thickening considerably as it does so. ‘One is tempted to regard 
these siliceous beds as representing a paleeozoic series of deposits of 
siliceous mud, derived, it may be, from the exuvize of the diatoms, 
radiolarians, and sponges inhabiting the deep sea at that remote 
period. Prof. Huxley (Contemp. Review) has shown how deposits 
of this nature may, by the partial—or the total—dissolution, and the 
subsequent redeposition of their siliceous particles, be converted into 
amorphous opaline silica. Such deposits as have lately been brought 
under the notice of the scientific world as one of the results of the 
like those above referred to, if there were frequent oscillations of 
level bringing about alternate deposits of siliceous mud and 
calcareous ooze’ (see J. G. Goodchild, Trans. Cumb. and Westm. 
Assoc., Part vii, p. 125, May 1882). This view has since been con- 
A remarkable plexus of faults occurs in Ousnop Beck, and the 
geological structure of that part is further complicated by other dis- 
turbances of the strata. The broader features of these were briefly 
described en route. The outcrop of the Main Limestone attracted 
much attention, as it forms a fine bold precipice on the east bank of 
the stream. In this scar Ravens (Corvus corax) bred up to within 
the last few years, and, doubtless, would do so still if they were left 
unmolested. 
Several of the party were much interested in some curious speci- 
men of (Main) Limestone which had been irregularly corroded by 
the action of surface waters, and subsequently glazed by a thin film 
of transparent calcite, redeposited from solution, during the drier 
part of the summer months. 
At Spout Gili attention was directed to such of the broader 
features of Swaledale geology visible from that point. The general 
north side of Thwaite, Muker, nnerside, etc., was pointed 
rigg and Muker Limestone lay at the bottom of 
the dale before the party, but the n downthrow of the 
ockdale Vein has let down much higher strata against the rocks 
ed. 
Naturalist, 
