470 C T. CLOUGH ON THE SECTION AT 



to be bounded ; and in either of the two unseen the state of things 

 may be very different from that seen in the others. 



These subordinate beds of basalt have been spoken of as under- 

 flows and overflows ; in every ease observed, however, they thin 

 out so very rapidly that it might be better to call them simply 

 wedges. They appear to belong in every way to the main mass of 

 basalt ; and probably one of the ordinary methods of the thinning 

 out &c. of the basalt is the giving off of these separation masses, and 

 their then dying out. Thus the thinning out &c. of the basalt as a 

 whole might probably be best represented, diagrammatically, some- 

 what as below : — 



Fig 2. — Thinning-out of the Basalt at the High Force. 



a. Sedimentary rocks. b. Basalt. c. Sedimentary rocks. 



I do not wish to imply by the above diagram that there is one 

 point from which the basalt can be made out to be thinning away 

 in every direction, but simply that, where the basalt is thinning 

 out, the thinning-out probably takes place in some such way as re- 

 presented. 



Prof. Sedgwick, in his paper on the Geology of High Teesdale 

 (Cambr. Phil. Trans. 1824), brought forward evidence which was 

 quite conclusive as to the truly intrusive character of the whin sill. 

 Prof. Phillips, at the time he wrote his ' Geology of Yorkshire,' does 

 not seem to have loyally accepted this conclusion, but to have been 

 more impressed with its supposed " general conformity over a vast 

 area to the beds above and below." It is this partial conformity 

 which still causes it to be an article in the miner's creed that the 

 great whin mass isa " sill," a bed regularly interbedded with the 

 other beds or " sills." On close examination of the ground, however, 

 it is seen that this conformity is not nearly so general as is supposed, 

 and that even in cases where the whin, as a whole, does keep on the 

 same geological horizon, there are numerous instances of small 

 subordinate beds separating off from the main mass in the manner 

 already described, and conclusively proving its intrusive character. 

 The High Force is one of the four localities specially mentioned by 

 Phillips (' Geology of Yorkshire,' part ii. p. 77) as showing the con- 

 formity between the surfaces of the whin sill and the surrounding 

 beds. We have seen that, instead of showing this, it affords striking 

 evidence to the contrary. The bed of the Tees near Winch Bridge 

 is another of the localities mentioned. This, too, shows good evi- 



