Yol. 55.] FAULTED INLIER IN TIDESWELL DALE. 249 



Specimens from the lower lava-flow contain two generations of 

 felspars, small pseudomorphs of olivine, and small crystals of a 

 slightly dichroic mineral which may be a pseudomorph of olivine. 



y. SUMMAET AND CONCLUSIONS. 



The toadstone of Tideswell Dale consists of lava-flows, between 

 which a sill has been intruded. This sill, which is from 60 to 70 

 feet thick, cuts across the lower portions of the lava, and sometimes 

 reaches the limestone below, but never approaches near enough to 

 the limestones above the lava to metamorphose them. The extent 

 of alteration of the subjacent limestone and the clay in any place 

 depends mainly upon the proximity of the sill to them. 



The alternative explanation, namely, that the whole mass is a 

 lava-flow, has for many years been accepted as correct. Against this, 

 it has been urged by Sir A. Geikie that such alteration of the 

 underlying rocks is rare in the case of lava-streams, but frequent in 

 the case of sills, and I may add that in no case of an undoubted 

 lava-stream in the county have 1 been able to find any marmorization 

 of the underlying limestone. The structure of the compact dolerite 

 or sill is essentially diff'erent from that of the lava above and below 

 it, and it has a greater specific gravity. Its hardness, absence of 

 vesicles, and coarse ophitic structure, passing gradually into almost 

 a basaltic rock, are characteristic of an intrusive mass. In a lava- 

 flow, however compact the central portions might be, we should 

 expect to find the slaggy under-surface continuous, but not inter- 

 rupted suddenly, and its place taken by the compact portion, as is 

 the case in Tideswell Dale. 



At some time posterior to the intrusion of the dolerite among 

 the lavas, the area between the two faults has been uplifted, and 

 later still the valley of Tideswell Dale has been eroded through the 

 igneous rocks, and through the limestones above and below them. 



EXPLANATION OF PLATES XIX & XX. 



Plate XIX. 



Geological map of Tideswell Dale and the neighbouring area, on the scale 

 of 6 inches to the mile. 



Plate XX. 



[The figures were photographed by the author under the microscope in 

 ordinary light, and enlarged 50 diameters.] 



Fig. 1. Ophitic olivine-dolerite from the top of the marble-quarry which is 

 about the centre of the sill. Shde No. 888. (See p. 246.) 



2. Olivine-dolerite, with large felspars and granular augite, 14 feet below 



the top of the quarry. Slide No. 887. (See p. 246.) 



3. Fine grained clivine-dolerite near the bottom of the sill. Slide No. 34. 



(See p. 246 ) 



4. Olivine-dolerite from the lava-flow at the top of the sill. (See p. 247.) 



5. Olivine-dolerite from the uppermost of the two lava-flows south of the 



inlier. (See p. 248.) 



6. Tufi" near the northern fault. (See p. 248.) 



