648 C. CALLAWAY ON THE PRECAMBRIAN 
latter sometimes displaying the characteristic banding of some of 
the Wrekin felstones. Fig. 2 shows the structure of the chain at 
this point. 
Fig. 2.—Section through Lawrence Hill. 
N.W. S.E. 
1. Bedded Precambrian tuff, dipping north. 
2. Quartzite. 
3. Hollybush Sandstone. 
a', a, a2, Doleritedykes. The position only of a? is indicated, and only 
so much of a? as is actually exposed is drawn. 
The shaded portion represents the strata exposed in the quarry. 
The continuation of the bedding beyond the quarry is indicated by 
the broken lines. The beds plunge into the face of the quarry 
obliquely, so that the section is taken at an acute angle with the dip, 
and the true dip is notshown. ‘The dyke @’ is well exposed, and, in 
its lower part, stands out like a wall. ‘The dolerite is fine-grained, 
and may be called basalt. It is considerably altered, calcite being 
deposited in the cracks, and the tuff in the vicinity of the dyke has 
also undergone more than its usual alteration. The changes in 
both cases I conceive to be due to the infiltration of carbonated 
waters subsequently to the injection of the basalt, the dislocation 
allowing passage to the water more freely than the unbroken strata. 
The altered tuff in proximity to the Lawrence-Hill dyke contains 
greenish matter, apparently some form of magnesian silicate, one of 
the commonest products of wet chemical decomposition. The 
surface of the dyke in contact with the tuff is minutely jointed into 
rectangular prisms an inch or two square, and lying with their 
long diameters at right angles to the bounding surface. I have 
detected two of these dykes, apparently a? and a*, on the Wrekin 
side of the ravine, in the road up to the cottage, and striking 
towards a mass of dolerite which protrudes at the surface within 
half a mile to the south-west in the summit ridge of the Wrekin. 
Fig. 2 also shows that on the south-east the tuff is overlain at a 
considerable angle by beds of quartzite, the former dipping north, 
the latter south-east. The quartzite is succeeded by the Hollybush 
Sandstone, and the Hollybush Sandstone by the Tremadoc Shales 
of Shineton. The planes separating all four formations from 
each other are probably strike-faults. The quartzite also occurs, 
but inconspicuously, on the north-west side of Lawrence Hill, dip 
undetermined. 
Summary.—A south-west ridge, composed mainly of felstone, but 
displaying at the south-west end a considerable thickness of 
