86 =—sO W. XW. Fontaine—Mesozoic Strata of Virginia. 
illustrate this, I may mention two cases. Lyell states that the 
Midlothian (old) shaft was sunk within the field to the west of 
the previous workings, and entered the coal three hundred feet 
higher than was expected from the dip, thus giving an upthrow 
of this amount. This is on the east side of the field. On the 
west side at the Dover Mines, the company owning the works 
attempted to develop a new portion of the field by sinking a 
shaft a few hundred yards to the east of their old workings. 
They penetrated the Sati¥e series of strata, and found nothing 
workable. : 
My examination of some of the finger-like remnants of the — 
Mesozoic, now found at the northern end of this field, thrust 
out in the Azo oic, put me in possession of what I think is the : 
Sy sere: of the peculiarities of the structure of this field, : 
the interior belts) The history of these areas, briefly j 
stated seems to be as follows :— ; 
The strata were laid down in depressions, which, originally _ 
shallow, were subsequently deepened by a more or less rapid _ 
subsidence. The subsidence was due , as previously stated, to 
the operation of a lateral thrust. It contintied until faults and 
overturned anticlinals were produced. In the interior belts — 
these operated to produce a constant seat wfat dip. This | 
resulted from the fact that the western sides of the severed earth 
prisms dropped, producing sometimes by a roll of the prisms _ 
an upthrow of the eastern side. This appears to occur in some _ 
of the faults of the Richmond coal field also. When the ~ 
strain did not result in producing rupture and faulting, it 
caused the development of an anticlinal, affecting but a nar- 
row belt, which was overturned to the eastward, thus produc- 
ing also a continuous northwest dip. here the strata have _ 
suffered enormously from erosion, and where almost Bostibl 
thickness of keg 
ae the Richmond oi field the faults and narrow overturned 
initenior belts, continuous di ips, but suffice only to‘render very 
variable and uncertain the dip and position of the strata 
toward the center of the field. The general result seems t 
of t 
the overturned antislindls are of extremely Hinited extent. 
have seen them only a few feet wide. 
