Prof. T. G. Bonney—Pebbles in the Bunter Beds. 199 
7. Thus in the gently undulating rocks of the Lowland regions the 
visible dips of the strata are reliable indices of the original relations 
of the sediments (Orthoclinic strata) ; but in intensely folded moun- 
tain regions the dips have been generally inverted, and give false 
ideas of the original sequence of the beds (Pseudoclinic strata). 
(b.) Special Principles. 
8. In Orthoclinic or slightly folded rocks, each fold or undula- 
tion (amphiplex or normal fold) is composed of two members, an 
arch or anticlinal, and a trough or synclinal. In both of these the axis 
is vertical, and the strata dip in opposite directions (Orthoplexal or 
amphiclinic strata) (Fig. 7). 
9. Where the compression increases in amount of force and 
obliquity of direction, as in the flanks of mountain chains, the amphi- 
plex, or normal fold, becomes gradually transformed into an oblique 
sigmoidal or S-shaped fold (sigma-flexure, sigmaplex or overfold), both 
axes of which are inverted (Fig. 10), and its beds all dip in one and 
the same general direction (Plagiopleral or isoclinic strata). 
10. Hach sigmaplex or sigmoidal fold is also composed of two 
’ members, an inverted arch or anticlinal, and an inverted trough or 
synclinal, having an intermediary wall or partition which is common 
to both. Its various parts may be conveniently distinguished as 
follows (compare Heim, Mechanismus der Gebirgsbildung, taf. xvi. 
Fig. 2) :— 
Terminology of a sigmaplex or sigmoidal fold (Overfold of Brégger) 
(Fig. 10). 
A. The arch or anticline. 
A.’ Core of the arch; A.? Crest of the arch; A.? Roof, or outer 
limb of the arch. 
B. The Partition, or common limb. 
C. The Trough or syncline. 
C.! Core of the trough; C.? Base of the trough; C% Floor, or 
outer limb of the trough. 
a, Sigmal or Plexal axis; y, Anticlinal or Arch axis; z, Synclinal 
or Trough axis. 
(To be continued.) 
Il.—Sreconp Note on tHE Prssues IN THE BunrerR BEDS OF 
STAFFORDSHIRE. 
By Prof. T. G. Bonney, M.A., F.R.S., F.G.S. 
N my brief paper published in this Magazine for 1880 (Decade II. 
Vol. VII. p. 404), I mentioned that pebbles of felstone were 
not uncommon in the Bunter conglomerate on the northern part 
of: Cannock Chase. Since that time, as opportunity has occurred, 
I have been making a more special study of these pebbles, and think 
it may be worth while publishing a description of some of the com- 
moner varieties, as a contribution to the lithology of this interest- 
ing deposit and a help to the determination of the question of the 
origin of its material. I believe that I have observed most of the 
varieties, which: commonly occur in the district, but do not pretend 
