374 EAETH-M0VE3IEXTS DURING CARBOXIFEllOUS TIMES. 
of the Craven faults during the Carboniferous Limestone* period 
served to sharply differentiate the rocks north and south of the 
faults, and proving that subsidence was greater in the southern 
area. It seems estabhshed, therefore, that after the Sikirian 
tloor had found relief from the tangentive stresses by which it 
had been affected, it came under the influence of normal gravita- 
tional stresses acting in a vertical plane, but differential in their 
effect on the rate of subsidence. 
III. — Datum Lines from which to Measure Vertical 
Movements. 
There is often some difficulty in deciding whether earth- 
movements in a vertical plane were positive or relative, or even 
to decide on the direction in which the movement took place, 
with the result that descriptions of earth-movements are often 
not so clear as could be desired. When possible, of coarse, it 
is desirable to decide positively whether the motion, in the case 
of vertical movements, was towards or from the geometrical 
centre of the earth. Lack of data renders this difficult some- 
times. The author therefore sought for some datum line in the 
Coal Measures themselves by which the character and extent of 
the movements affecting those rocks could be determined. He 
took the more persistent coal seams, such as the Barnsley and 
Kents Thick, and assumed, as seems generally accepted, f that 
they were laid doAvn in marshes or lagoon-hke areas, and that 
during formation they were approximate^ in a horizontal plane. 
If this be granted, then we have at our disposal an approximate 
datum line from which to measure vertical movements. If we 
take the Barnsley seam, use it as a datum hne, and, neglecting 
the faults, plot down the position which the older seams then 
occupied, it becomes very evident that sedimentation had been 
accompanied by differential subsidence, as wdll be seen on refer- 
ence to Fig. 4, Plate LI, while several interesting modifications 
are seen towards the margins. 
R. H. Tiddeman. Brit. Assoc. Rep., 1889, p. 600. 
t Strahaii. Brit. Assoc. Rep., 1900, p. 746. 
