^^1* 55'] FAULTED UGLIER IN TIDES WELL DALE. 243 



vesicular toadstoiie at the eastern end of the section dip nearly due 

 east. 



The mass of compact dolerite, therefore, occupies different horizons 

 in the lava-flow, and must be intrusive. Further evidence of 

 intrusion has been obtained by an examination of the limestone- 

 beds immediately below the toadstone, on both sides of the valley. 



Some 7 feet below the clay is a well-marked bed of finely- 

 laminated limestone, several inches thick. This is easily traced on 

 the western side of the valley for a distance of nearly 300 yards 

 along the middle road to Tideswell, and is also seen in the marble- 

 quarry on the opposite side of the valley. This bed was taken 

 as a datum-line, and at various points (marked A to M on the 

 section, fig. 2, p. 244) measurements were made of the thickness of 

 the limestone and clay above it, and the depth to which marmori- 

 zation of the limestone extended. The results were plotted to 

 scale on a diagrammatic section, in which all effects of the minor 

 faults and of the undulations of the beds were eliminated by 

 making the datum-line horizontal. The length along the horizontal 

 line gives the position of any place in the section, while the vertical 

 distances indicate the thickness of the beds and the extent of 

 marmorization. 



The marmorized limestone is generally white, has a translucent 

 surface when wet, breaks with a saccharoidal fracture, and is 

 easily crushed into a white crystalline powder. 



(1) The thickness of the limestone above the datum-line is 

 practically constant, varying from 7 to 7^ feet, except for a short 

 distance at the southern end (where the rocks are covered with 

 soil) ; and consequently the compact dolerite does not cut across the 

 beds of limestone. 



(2) The clay varies in thickness from 7 feet downward, and in 

 places is absent altogether. 



(3) Where the non-vesicular and compact dolerite rests upon the 

 clay, or upon the limestone, the clay is rendered columnar and the 

 limestone is marmorized to a depth down to or below the datum- 

 bed. 



(4) Where the toadstone resting upon the limestone or clay is 

 vesicular or slaggy, and attains a sufficient thickness, the clay and 

 the limestone are not altered. 



(5) The bases of the compact dolerite and of the marmorized 

 limestone are approximately parallel. 



These deductions point to the conclusion that the metamorphic 

 effect is due to the compact dolerite, and not to the vesicular rock. 



This varying amount of metamorphism, due to varying proximity 

 of the intrusive rock to the limestone below it, is seen in other 

 parts of the dale. On the southern side of the quarry the limestone 

 is unaltered, while on the northern side it is marmorized to a depth 

 of several feet, and the compact dolerite is found resting upon it. 

 The marmorization may be traced from the quarry on the eastern 

 slope as far as the cottages north of the quarry. Near Meadow 

 Farm, where there is a great thickness of the vesicular rock beneath 



e2 



