the Basaltic District in the North of Ireland, 263 



at some distance, and can often distinctly trace the bases to 

 three or four different strata at the same time, as I ride 

 alongj sometimes for miles together, before any pits or 

 quarries are found, open,, by which to identify the substances 

 of which they severally consist. 



I mention the above, both as a circumstance of great 

 practical importance in mineralogical surveying, and also as 

 tending to prove, that the strata had acquired their present 

 comparative hardness, before the denudation and excava- 

 tions spoken of, took effect : the faults likewise have hap- 

 pened since the consolidation of the strata, as their ground 

 edges in numerous instances prove, and the disarrangement 

 they occasioned in the strata must likewise have occurred^ 

 prior to the final denudations and excavations of the surface, 

 since very few of the numerous faults which raise the mea- 

 sures on one side or depress those on the other, are visible 

 by any inequalities on the surface, except that a very atten- 

 tive and experienced eye may often discover their situation, 

 by means, of the interruptions they give to the faint tablets 

 of strata, above described : by which, I have sometimes 

 greatly surprised practical miners in tracing out the principal 

 faults of their district : a circumstance often of the utmost 

 importance in practical mining. 



I cannot conclude this letter, without heartily congratu- 

 lating Dr. Richardson on the very great progress which he has 

 made in these inquiries, and expressing a hope, that he will 

 still persevere; endeavouring also to bring fresh labourers 

 into the field, for the purpose of giving us a general and 

 connected idea, of the order and position of all the principal 

 strata of the interesting island wherein he resides. Maps 

 also, showing by different colours, the surface occupied by 

 each particular stratum, and vertical sections in particular 

 directions similarly coloured, are much wanted, and will, I 

 hope^ ere long, be undertaken. 



I am, sir, your obedient servant, 



JOHN FaRKY, Mii}eralog?c<lSurveycr. 



12, Upper Crown Street, Westminster, 

 April 5, 1809. 



R4 XLVI. Ana- 



