272 MR. H. H. AENOLD-BEMEOSE ON [Aug. I907, 



Laneside Farm. The intrusive rock extends therefore under 

 Snelslow, and measures two-thirds of a mile from north to south 

 and half a mile from east to west. North of Snelslow it is in places 

 an ophitic, and in others a fine-grained, dolerite, similar to that near 

 the upper margin at Mill Cottage. The sill is thickest near Mill 

 Cottage, and the contact-alteration in the limestone above it is 

 greatest there. 



Several thin slices of the marginal rock were examined under 

 the microscope. One specimen contained small augite-plates (seldom 

 penetrated by felspars) and small prisms of augite. It is a transition 

 between the ophitic and the granular type of intrusive rock. About 

 1 foot below the limestone the rock consists of a much-altered 

 dolerite, of which altered plagioclase-laths and secondary silica in 

 veins form the bulk. The upper margin consists of an altered 

 olivine-dolerite with granular augite, traversed by veins of crystal- 

 line silica. 



Potluck Sill (PI. XIX). — About a mile and a third east of the 

 village of Peak Forest and two-thirds of a mile north of Wheston, 

 big blocks of a coarse-grained dolerite project above the surface of 

 the ground over an area of nearly 90 acres. South of Potluck in 

 the surface-soil are numerous blocks and spheroids of a partly- 

 decomposed dolerite, on the weathered surfaces of which are seen 

 the reddish-yellow pseudomorphs of olivine possessing the charac- 

 teristic outlines and cracks of that mineral. 1 The intrusive sheet 

 rests upon the limestone-beds to the west, which dip under it at an 

 angle of 10°. Its eastern boundary cuts across the strike of the 

 limestones near the road between Peak Forest and Tideswell, and 

 in the small valley to the north of Wall Cliff. On the northern and 

 southern boundaries it has pushed its way up to the lower lava of 

 the Miller's-Dale area. Although there are no well-marked signs 

 of contact-alteration in the limestones above the sill, big blocks of 

 marmorized limestone lie in a field to the north ; and in two small 

 quarries the limestone is crystalline, and has a powdery fracture. 

 In the north-western portion of the sill are several small quarries y 

 in which blocks of the igneous rock are seen in a clay on the surface 

 of the limestone below. The sill is here very thin, and no altera- 

 tion of the limestones below it has been detected. A measurement 

 of the beds shows that near the northern boundary the igneous rock 

 is less than 50 feet thick. 



It is interesting to refer to Whitehurst's information on the 

 thickness of the toadstone-beds in the immediate neighbourhood. 2 

 He states that at Black Hillock (a shaft near New Farm, on the 

 south-eastern boundary of the northern limb of the sill) a shaft was 

 sunk 100 fathoms in the toadstone and the bottom was not reached, 

 and that its thickness varied from 2 to 19 fathoms in seven other 

 mines north and north-east of the Hillock. In Chapmaiden Mine, 



1 Quart. Journ. Geol. Soc. vol, 1 (1894) p. 613 & pi. xxiv, figs. 2-4. 



2 ' An Inquiry into the Original State & Formation of the Earth ' 4to. 

 London, 1778, p. 161 & pi. vii. 



