138 



ISOLATED CAPS OF BASALT. 



wick) will give a distinct idea of this mode of formation. It may- 

 be proper to observe, that the dyke is a continuation of the Cleveland 

 basalt dyke, which I have before described. The horizontal meas- 

 ures through which it passes are coal measures. 



There can be no more doubt respecting the cap or expansion of 

 basalt having been erupted through the dyke, than there can be of the 

 origin of a bed of lava, which may be traced to the mouth of an ad- 

 jacent volcano. 



Beds of trap or basalt, interstratified with other rocks, have given 

 rise to much speculation respecting their origin : that such beds are 

 not unfrequent in the coal measures, is a fact well known to miners in 

 the North of England. From the great hardness of trap beds (pro- 

 vincially called beds of whinstone) ihey increase the difficulty and 

 expense of sinking shafts. These interstratified masses have been 

 frequently described, as regular measures or strata. There is a 

 thick bed of trap in some of the coal-fields in Durham, called the 

 Great Whinstone Sill; the word sill being used for stratum by Mr. 

 Westgarth Forster, in his section of these strata published in 1809. 

 This bed or mass of whinstone, though described by Mr. Forster as 

 a regular stratum with the series of strata in which it is found, is ad- 

 mitted to vary in thickness from twelve to sixty yards, it is found 

 at a great depth in some mines ; in other situations it rises to the sur- 

 face. Upon the supposition that this bed is of igneous origin, the 

 question has been proposed, in what manner did it become interstrat- 

 ified with beds that are evidently aqueous depositions? Those who' 

 first raised this objection could scarcely have kept in mind, that every 

 bed in the whole series of the coal measures was once the upper sur- 

 face of the solid ground, whether that surface was covered with wa- 

 ter, or was dry. An eruption of lava might therefore flow over any* 

 particular bed in the whole series, and this lava might become cov- 

 ered by subsequent aqueous depositions. But there is another mode 

 in which the lava might be introduced among the strata at a later 



