50 NOTES ON CRICH HILL. 



though the shapes of the oHvine and felspar crystals remain. In 

 addition, the rock contains a quantity of pyrites, scattered 

 throughout the mass. 



I had small pieces of this rock from different parts of the bed 

 cut, ground, and polished until they became so thin as to be 

 transparent. When these are placed under the microscope, the 

 minute structure of the rock is seen. The following is a very 

 brief description of the specimens thus examined. 



(i) Toadstone in- which the ore occurs. The felspars appear 

 in the section like laths or matches. They are much altered. 

 The black material is pyrites, and is not an original constituent. 

 The rock, technically speaking, is a decomposed olivine-dolerite. 



(2) Upper part of toadstone bed, called Blacks tone by miners, 

 within a few yards of the limestone above it. An altered olivine- 

 dolerite, now consisting of felspars, all altered to calcite and 

 serpentine, olivine altered to the same materials, small rods 

 of magnetite or ilmenite, the whole traversed by veins of calcite 

 and haematite. 



(3) Lower dotvn, " Blackstone." Less altered than No. i. 

 The felspars are fresher. The olivine is altered to calcite. 

 Much calcite occurs throughout the mass, and probably replaces 

 augite. 



(4) Near the bottom of bed, called toadstone by the jniners, 

 a light-grey hard rock, of similar structure to No. i. Much more 

 altered than Nos, 2 and 3. 



The rock in the interior of the bed is, therefore, less altered 

 than that near the top and bottom. 



(5) From mine heap near the top of the hill is less altered than 

 any of the preceding. The felspars are nearly fresh, the olivine 

 is altered to calcite ; a little fresh augite occurs, magnetite is 

 also present. 



This toadstone was, no doubt, once a lava stream. There 

 are several beds of this rock in Derbyshire, and I hope in a 

 future volume of the /ournal to describe some of them, and 

 point out the evidence we have of former volcanic action in 

 our county. 



