>20 



TESTS OF RELATIVE AGE 



[Ch. XXX 



Fig. 658. 



if newly thrown down, they thin out in certain places, thus allowing the 

 lava to cross their edges. Besides, the heavy igneous fluid will often, as 

 it moves along, cut a channel into beds of soft mud and sand. Suppose 

 the submarine lava f, fig. 658, to have 

 come in contact in this manner with the 

 strata a, b, c, and that after its consolida- 

 tion, the strata d, e, are thrown down in a 

 nearly horizontal position, yet so as to lie 

 unconformably to f, the appearance of 

 subsequent intrusion will here be com- 

 plete, although the trap is in fact con- 

 temporaneous. We must not, therefore, hastdy infer that the rock f is 

 intrusive, unless we find the strata d, e, or c to have been altered at their 

 junction, as if by heat. 



When trap dikes were described in the preceding chapter, they were 

 shown to be more modern than all the strata which they traverse. A 

 basaltic dike at Quarrington Hill, near Durham, passes through coal- 

 measures, the strata of which are inclined, and shifted so that those on 

 the north side of the dike are 24 feet above the level of the correspond- 



Fig. 659. 

 Magnesian limestone. 



Coal. 



Section at Quarrington Hill, east of Durham. (Sedgwick.) 



a. Magnesian Limestone (Permian). 6. Lower New Red Sandstone. 



c. Coal strata. 



ing beds on the south side (see section, fig. 659). But the horizontal 

 beds of overlying Red Sandstone and Magnesian Limestone are not cut 

 through by the dike. Now here the coal-measures were not only depos- 

 ited, but had subsequently been disturbed, fissured, and shifted, before 

 the fluid trap now forming the dike was introduced into a rent. It is 

 also clear that some of the upper edges of the coal strata, together with 

 the upper part of the dike, had been subsequently removed by denuda- 

 tion before the lower New Red Sandstone and Magnesian Limestone 

 were superimposed. Even in this case, however, although the date of 

 the volcanic eruption is brought within narrow limits, it cannot be defined 

 with precision ; it may have happened either at the close of the Carbo- 

 niferous period, or early in that of the Lower New Red Sandstone, or 

 between these two periods, when the state of the animate creation and 

 the physical geography of Em-ope were gradually changing from the type 

 of the Carboniferous era to that of the Permian. 



