TRANSACTIONS OF SECTION C. 801 
sharply angular outlines, some few are exceptionally well rounded and water-worn, 
These detrital products, enclosed in a volcanic ash matrix of a diabasic character, 
have consolidated mto a tough rock of low specific gravity (2-693), possessing 
great tenacity of resistance to abrasion. With the exception of a few strain- 
shadows in quartz grains, the microscopic slides exhibited offer no suggestion of 
the ingredients having suffered from mechanical deformation ; but in the quarry 
may be noted one or two examples of schistosity resulting from shearing, these 
being restricted locally to the proximity of shrinkage joints now filled in with 
quartz and an earthy, green, derivative product. Above and below, this rock 
shades off into indurated grey-green ash beds, Flakes and lenticular fragments of 
volcanic mud scooped off the old sea floor have been caught up in the superposed 
volcanic tuff near to the line of junction, and some of these entangled patches, 
when freshly exposed in the quarry, show a septarian arrangement internally, the 
outer portions flaking off along faces coated with a lustrous film, the inner surface 
subsequently weathering to a variegated dull purple or brownish tint. 
The quarry may be inspected on the way to or from Ingleborough. 
FRIDAY, SEPTEMBER 5. 
The following Papers and Reports were read :— 
1. The Devonian Rocks, as described in De la Beche’s Report, interpreted in 
accordance with Recent Researches. By W. A. E. Ussuer, F.G.S. 
[Communicated by permission of the Director-General of the Geological Survey.] 
Owing to the very complex association and variable characters of the Devonian 
rocks of the South-Western counties, the information gleaned by Sir Henry De la 
Beche, during a very rapid survey made more than fifty years ago, did not enable 
that eminent pioneer of Stratigraphical Geology to arrive at any certain conclu- 
sions respecting the relations of the strata composing the then-called Grauwacke 
System. The results obtained by a careful study of Chapter III., on the Grauwacke 
System, in De la Beche’s Report, are most unequal. 
Where the structure is comparatively simple, as in North Devon, the succession 
is given (pp. 45-56) in a plain and masterly manner; and although no classification 
is put forward, the strata are described in successive groups, each of which corre- 
sponds to a true subdivision. The grouping I have adopted for North Devon, by 
mapping out the subdivisions in the field, is De la Beche’s grouping accentuated 
by names and geological boundaries. He applies the same grouping to West 
Somerset, where the structure is much more complex, and his correlations are correct. 
Sections I. and II., Plate III., are admirable illustrations of the succession of the 
Devonian subdivisions. 
Turning to the intricate and involved region of South Devon (pp. 64-78), 
we find that the grouping is based on the assumption that strike-lines have the 
value of horizons, and thereby the South Devon limestones are made to occupy 
several distinct horizons in the slates. Although contemporaneous voleanic action 
is pointed out, yet the greatest tract of voleanic rocks in the whole region (z.e., 
the Ashprington Series) is confounded (p.76) with arenaceous rocks (now known 
to be Lower Devonian). Inverted junctions are regarded as natural junctions, as 
in the Plymouth succession (p. 65). As De la Beche’s suggested correlations apply 
to an interpretation of this part of the area supplied by co-workers, the reader must 
not hold him in any way responsible for them. 
The treatment of Cornwall differs from South Devon, with which itis in many 
places so interwoven as to render it difficult to follow the text. Here we appear 
to have the strivings of the great geologist to piece together and simplify the results 
of his direct personal observations, That he failed is due, not only to insufliciency of 
material, but to the absence of allowance for inverted junctions ; again and again 
he is confronted with anomalous appearances of this kind, so that his correlations, 
