British Mineralogy . 349 



pcarance totally different from any of the clays occurring either 

 in the Carboniferous or Silurian formations of the neighbourhood, 

 or even likely to have resulted from the alteration of any of the 

 known strata in these formations, made the author conclude that 

 this clay must have proceeded from the decomposition of the 

 eruptive basaltic rocks themselves. Chemical analysis was re- 

 sorted to in order to obtain evidence in support of or against 

 this view, and appeared to be quite conclusive in its favour, since 

 not only did the results of the analysis show that there was 

 nothing, from a chemical point of view, to militate against the 

 supposition that this clay had been of such origin, but the 

 amount of titanic acid which it contained seemed to decide the 

 point most satisfactorily. 



In another instance, in the same district, at the coal-pit No. 7, 

 Uighlanes, the thick coal was found to be traversed by a dyke of 

 a white rock (the so-called .white horse of the Staffordshire 

 miners) which broke through it and sent ramifications into the 

 coal. 



The coal at points of contact with this rock was evidently 

 greatly altered, having lost most of its volatile ingredients, and 

 become, as it were, charred and nearly incombustible. The 

 rock, on the other hand, so far from possessing the normal ap- 

 pearance of the eruptive dolerites of the vicinity, appeared in ex- 

 ternal characters to differ but little from a somewhat indurated 

 greyish-white clay, and at first sight would be taken for any- 

 thing but igneous. The chemical analysis showed, its composi- 

 tion as follows : — 



White-horse rock, pit No. 7, Uighlanes. Specific gravity 2*81 . 



Silica. . ■ 31-39 



Titanic acid .... 1*02 



Alumina 27'80 



Lime. ...... 2-52 



Magnesia 1*66 



Soda 0'54 



Protoxide of iron . . . 16-13 



Protoxide of manganese . 009 



Water 19-62 



100-77 



with traces of carbonic acid and potash, proving most satisfacto- 

 rily its identity with the ordinary dolerite of the district, which 

 evidently in this case had been altered in situ by the action of 

 water. The microscopic examination also showed that enough 

 of the crystallized structure still remained to prove its origin*. 



* " The Microscope in Geology," Popular Science Review for October 

 1867, pi. 18. fig. 12. 



