12 MR. H. c. SARGENT OX LOWER carbo:niferous [yoL Ixxiii, 



II. The Latas 



The features of the Tideswell-Dale rock that dh-ected attention 

 to the subject were {a) the abundance of oUgoclase among the 

 i'elspathic constituents : (b) the occurrence of albite in the vesicles; 

 and (c) the subordinate part played by augite as compared with 

 olivine. Many specimens from other areas of the county have now 

 been examined under the microscope; and, although from numerous 

 exposures, Dr. Bemrose's petrographical description ^ is fully borne 

 out, there are many from various localities which possess features 

 similar to those of the Tideswell-Dale rock. Such localities as have 

 been hitherto noted by me are set forth in the accompan3^ing list, 

 and it will be seen that all the main flo\vs of the county, as mapped 

 by Dr. Bemrose, are represented, as well as several of those which 

 he describes as ' uncorrelated.' " With the exception of Kniveton, 

 all the localities are in the Carboniferous Limestone. 





Locality. 



Hovizon 





1. 



Ball Eye. Via Gellia, near Cromford. ~" 





"Upper Lava. 



2. 



Salter's Lane. Matlock 



■ 



Do. do. 



3. 



Masson. Matlock 





Do. do. 



4. 



Dobb Lane, Bonsall 



> Matlock Area < 



Lower Lava. 



5. 



Ember Lane. Bonsall 





Do. do. 



6. 



Aldwark Grange, near Grange Mill. 





Uncorrelated. 



7. 



Sactiereral Barn, near Grange Mill. ^ 





Do. 



8. 



Knot Low. Millers Dale " 





'Upper Lava. 



9. 



Tideswell Dale 



>Millers Dale Area. < 



Lower Lava. 



10. 



Tideswell Dale 



Uncorrelated. 



11. 



New Bridge, near Ashford ^ 





Do. 



12. 



Conksbury Bridge. Lathkill Dale. 



Upper Lava of Lathk 



ill Dale. 



13. 



Bradford Dale, Youlgrave 



Bakewell Area .. 



Uncorrelated. 



14. 



Lumford Mill, near Bakewell 



Do. 



15. 



Crich Cliff 



Donbtfnl. 





16. 



Kniveton 



Tissington Area . . . 



Limestone 



Shales. 



In both macroscopic and microscopic characters the Derbyshire 

 lavas show a rather wide range. Their colour varies from different 

 shades of grey, green, or blue, to black. Their texture ma}' be fine- 

 grained or coarsely crystalline, Avith gradations from one extreme 

 to the other. 



The rocks with which this paper is concern'ed belong to the 

 leucocratic end of the series. They are nearly always vesicular 

 and always intense^ altered. Both olivine and augite appear to 

 have been invariably present in the original rock, the former 

 sometimes pre])onderating over the latter, as already noted in 

 the Tideswell-Dale specimen ; but, under the microscope, both 

 minerals are always seen to be completely destroyed. The 

 evidence of their former presence consists in a more or less com- 

 plete recrystallization, pseudomorphous or otherwise, of secondary 

 products such as calcite, quartz, and minerals of a chloritic or 



1 Q. J. G. S. vol. 1 (1894) pp. 611-25. 



2 Ihid. vol. Ixiii (1907) pp. 247 & 251. 



