270 Geological Notes on the Middleton-in-Teesdale District. 



granular fracture increased from the bottom upward, and that 

 the shales with which it had come into contact were greatly 

 altered. One of these, locally known as the ' pencil bed,' 

 was erroneously believed to be the same formation as the shales 

 near Cronkley, from which slate pencils were once made. The 

 property of being able to mark slate is common to both, but 

 on examining both shales carefully there is no hesitation in 

 confirming the conclusion of the Survey that the Cronkley 

 shales are probably of Silurian origin, and an upthrow of the 

 Burtreeford Dyke, and that the Park End shale is a deposit of 

 fine clayey matter formed during Yoredale times, which has 

 since been much altered by contact with the Whin Sill. The 

 analyses of the two shales are quite different. 



At Park End quarry is a very interesting fault. The out- 

 crop of the Whin abuts on indurated shale and adhering 

 to the Whin is a thin vertical sheet of hard scoriaceous material, 

 full of cavities. The true Whin is seen to interlock into this 

 ' crust ' in serrated edges, but the thin wall of crust shews clear 

 lines of stratification, and where it joins the Whin the lines of 

 stratification are very distinctly curved upward. 



Is this ' crust ' the result of water action in the fault after 

 the intrusion of the Basalt ? Or is it an actual crust of intru- 

 sive lava cooled against a surface of shale against which it has 

 come up ? Is the fault part of the great fault formed after the 

 intrusion of the Basalt ? Or is it a pre-existing fault ? Has the 

 sheet of Basalt been broken at this point and denuded, or has 

 it here cut across the bedding planes and risen to a higher level 

 through finding its course obstructed and then been denuded ? 



These questions are left unsettled, and may give an oppor- 

 tunity for some further discussion. 



The effect of contact with the Whin Sill which was noted in 

 this quarry, was seen to still greater perfection at High Force 

 where the stratified shales and grits and limestone below the 

 Whin have assumed a prismatic character. But perhaps the 

 most interesting section of altered limestone is at White Force, 

 where some thirty to fifty feet of underlying strata have been 

 completely marmorised, although on weathering the limestone 

 is reduced to fine granular white sand. In this section there 

 still remains evidence of the fossils which it contained prior to 

 its metamorphosis. Still further evidence of altered strata 

 occurs on Widdy Bank in the vicinity of Cauldron Snout, but 

 here it is the overl3nng limestone which has become granular 



Naturalist, 



