190 
JOURNAL OF THE PLYMOUTH INSTITUTION. 
The mean of the several measurements — all very near the average 
— gave a Dip of 12°*5 toward S. 35° W., magnetic. 
Three such Sections were made in this southern wall — the first 
near the western end of the Pit, the second 460 feet east of the 
first, and the third 220 feet still farther east. Confining attention 
mainly to the first, which, on account of certain local peculiarities 
was the most important and instructive, the Section disclosed, from 
the bottom of the Pit upward, 23 beds of Clay alternating with 
22 of Lignite. These were followed by a thick bed of Sand, above 
which were 13 beds of Clay, 9 of Lignite, and 3 of Sand, making 
a total of 36 beds of Clay, 31 of Lignite, and 4 of Sand. The 
Clay beds varied from 69 inches to 1 inch in thickness, and the 
Lignites from 74 inches to 1 inch. 
Above these was an accumulation of Clay and Sand 7*5 feet 
thick, with angular and sub-angular stones varying in size from 
that of a hen's egg to blocks from 6 to 7 inches in mean diameter; 
the whole being without any approach to regular or symmetrical 
arrangement, and overlying the Lignitiferous Series unconformably. 
This accumulation, locally known as " The Head," occurred in every 
part of the Basin, and was frequently of greater thickness than at 
the Pit. 
In certain parts of the Basin the "Head" was itself covered 
with well-marked beds of Clay and Sand without stones, having a 
total thickness of about 10 feet, and containing, near the bottom 
of the lowest bed, leaves of the Dwarf Birch ( = Betula nana, 
Linn.) and two or three species of Willow ( = SaMx, Linn.) 
It cannot be doubted that the deposits disclosed at and near the 
Pit represented three distinct Formations and Periods— the Ligniti- 
ferous Series, the " Head," and the Betula nana Clays ; and it is 
probable that these were separated by Periods unrepresented b^ 
deposits ; thus, and to go no further, the Period of the deposition 
of the Lignitiferous beds was certainly separated from that of the 
" Head " by an interval of disturbance, during which the former 
were all thrown into an inclined position. 
Confining attention henceforward to the Lowest Formation — 
the Lignites with their interstratified Clays and Sands — it is note- 
worthy that, with the exception of crystals of Felspar, to be noticed 
again subsequently, there were no stones, large or small ; and that 
all the detrital beds became thinner, and in some cases " thinned 
