Vol. 55.] A SILL AXD FAULTED INLIER ITT TlDEbWELL DALE. 2-Lo 



the intrusive dole rite, the limestone under it is not marmorized, 

 and there is no deposit of clay. 



I have been unable to lind any alteration in the limestone vi^hich 

 lies above these igneous rocks. This is probably due to the fact 

 that the intrusive rock made its way among the lower portions of 

 the lava, and that the superincumbent limestones were protected 

 by the greater thickness of lava between them and the sill. The 

 vesicular rock resting upon the intrusive dolerite is seen on the hill- 

 side some distance above the quarry, and can be traced from near the 

 southern fault, where it is at least iO to 20 feet thick, to the northern 

 fault, where it attains a still greater thickness. 



The vesicular rocks immediately above and below the sill are 

 sometimes very hard : this may be due to the heating effects of 

 the intrusive dolerite. 



Near the northern fault, on the western side of the valley, is a 

 small patch of tuff. It can be traced for a vertical distance of about 

 20 feet, and about the same distance horizontally. It is not a 

 banded tuff, but in places there are signs of a very rough bedding 

 dipping 50° northward. It may be traced to within a few feet of 

 the limestone- escarpment which bounds the fault on the north, 

 being separated from it by a slight depression in the ground, about 

 2 feet deep. It is much harder than the bedded ashes of the district, 

 and may either represent a small deposit of tuff bedded with the 

 upper part of the lava-flow, and indurated by the sill ; or it may be 

 a portion of a small neck of agglomerate, which has been faulted 

 against the limestone. At present, the evidence seems to me in- ' 

 sufficient for arriving at a definite conclusion. 



IV. Peteographt. 



{a) The Intrusive Dolerite or Sill. 



The compact dolerite in the quarry was described by Mr. Teall 

 as ' an olivine-dolerite which shows in certain parts the ophitic type 

 of structure, in others the type with granular augites. Indications 

 of fluxion-structure may be seen in the latter.' ^ 



Eighteen thin slices 0? the compact dolerite from various parts of 

 the mass were examined by me. In addition to the types described 

 by Mr. Teall, is one in which the felspars largely predominate over 

 the other minerals : they are present in two generations, and attain 

 a length of 2'd mm. 



Specimens were obtained from the quarry-face at different heights 

 in an almost vertical line, so as to ascertain what variation of structure 

 occurred : — 



No. 710. — In a specimen a few feet above the top of the 

 quarry (sp. gr. 2*80) the felspars form by far the greater 

 portion of the rock. They occur in two generations, the 



1 'Brit. Petrogr.' 1888, p. 210 & pi. ix, figs. 1 & 2. 



