142 



Correspondence — The Rev. H. G. Day. 



in plane sections, the ordinary rule of * foreshortening,' as taught in 

 all Schools of Art, will amply suffice. 



I have found in practice that a shadow cast by sunlight on a paper 

 properly inclined gives the true result most simply. It is, however, 

 worthy of remark that the outcrop of a cylindrical stratum on a 

 plane surface cannot differ from the outcrop of a plane stratum on 

 a cylindrical hill, or in a hollow cylindrical valley ; and is therefore 

 reducible to " Sopwith's models." 



Next, in assuming that the trail outcrop had a definite direction 

 on either side of the railway cutting, does not Mr. Fisher assume 

 that the trail lies in one plane ? 



Under these circumstances a straight rod placed at the one out- 

 crop parallel to the other outcrop satisfactorily determines the strike 

 and dip of the stratum. 



As regards the equation tan ^ = cos /S tan a (2) 



I subjoin a short proof. 



Let A B C Dhe horizontal (strike) lines in the inclined plane. 

 £ E vertical,- C DE a, horizontal plane. 



Then ^ B C E = (fi 

 BDE = a 

 CED =/3 

 Also CDE is right angle. 



^ ^ , BE BE COB ^ 



Hence tan <^ = -^ =^ri>~ 



= tan 



H. G. Day, M.A. 



THE PEE-CAMBRIAN EOCKS OF BRITAIN AND BOHEMIA. 

 Sir,— In Dr. Callaway's letter on this subject (Geol. Mag. Feb. 

 1881), there are some passages which are to my mind a little mis- 

 leading in regard to the Dimetian rocks at St. Davids. The main 

 portion of the group consists of what appears to be a massive 

 granitoid rock, but on closer examination traces of foliation are 



