4 PROF. T. G. BONNEY ON THE GEOLOGY 
which have been formed by a bar of shingle and sand damming up 
the streams issuing from one or more valleys. It is, however, on an 
exceptionally large scale, and is worthy of more attention from 
physical geologists than it seems to have received. In the headland 
bounding the northern side of this sheet of water grey satiny slates, 
not unlike those in the mid-Devonian series of the northern part of 
the county, are exposed*. The headland immediately south of 
Torcross consists of dark slaty and fine gritty beds thrown into great 
folds with many minor contortions. On its southern side is a thick 
mass of dark glossy slate, which is quarried for roofing-purposes. 
All these rocks have undergone little alteration other than mecha- 
nical. They are distinctly cleaved, the planes dipping, usually at 
high angles, to the N. or slightly to the W. of N., and the strike both 
of the cleavage and the bedding being not far away from E. and 
W. The petrology of this group is interesting in more than one 
respect; but this I shall reserve for a separate section (No. 7), in 
order not to interrupt the description of the general stratigraphy 
of the district. South of the slate-quarry just named is another 
valley, now occupied by a small ‘ Lee,’ and at the end of the bar 
and shore called Bossons Sand comes another headland, on the 
northern side of which nestles a hamlet bearing the same name. 
Here, according to the map of the Geological Survey, begins the 
tract of metamorphic rock which occupies the remaining part of 
S. Devon, the boundary of this and of the unaltered region running 
nearly due west by South Pool and Marlborough, until it finally 
reaches the sea at Hope Cove. At first sight there seems good 
reason for putting the boundary here on the eastern coast, and much 
evidence in favour of those who maintain that the rocks between 
Torcross and the Start Poimt exhibit progressive metamorphism. 
Indeed, on my first traverse of the ground, [inclined to the same 
view ; but after carefully reexamining the section from Bossons 
Sands to some distance south of the village of Hall Sands (similarly 
situated at the next opening in the uplands), I observed that the 
rocks in the headland between these two valleys, though locally they 
might assume the macroscopic aspect of schists 7, reverted again and 
within the last few years) in the next valley to the south. The Loe Pool, near 
Helston, is another ; and it is possible that the Chesil Bank at Portland may be 
the remains of a like phenomenon. IJ think it probable that these shingle beaches 
are memorials of a period when the coast was depressed slightly below its 
present level, and thus are bars once formed at the mouths of estuaries and 
since elevated. 
* Dr. Holl (Quart. Journ. Geol. Soc. vol. xxiv. p. 400) considers the whole 
series of this Torcross district as higher in position than the Plymouth Lime- 
stone, and thus equivalent to the upper part of the middle and lower part of the 
Upper Devonian of the north, 
Tt Throughout this paper I apply the term ‘“‘schist” (as is my constant prac- 
tice) only to foliated rocks, in accordance with the limitation insisted on by 
the late Professor Jukes. The lax use of the term by many continental and 
some English petrologists (by whom it is made to include not only highly meta- 
- morphic rocks, but also some where the chemical and mineral changes are but 
slight) causes much confusion, both in description and (as I haye observed) in 
reasoning. 
OO 
