332 



Correspondence — Mr. W. H. Dalton. 



surface published in your last Number. Let me add that the 

 striations, though rightly rendered as to direction in that lithograph, 

 are not given with sufficient fineness or regularity. I may refer 

 your readers to the plates of Scaphaspis Lloydii and the restorations 

 of the shield of Pteraspis in my monograph for accurate reproduction 

 of striations having the same character as those of Kolaspis. It is 

 especially with Scaphaspis that Holaspis agrees in the form and size 

 of its skin-like grooving. E. Kay IiAnkestek. 



Oxford, June ISth. 



GEOLOGICAL PROBLEMS. 



Sir, — The following methods of finding by diagram, in place of 

 calculation, certain data for the construction of correct geological 

 sections, may be found useful. 



Problem 1. — To find the apparent angle in any required section 

 from the full dip, and the deviation of its direction from that of the 

 section. 



Construct the right-angled triangle A B C, with ABC equal to 

 the full dip ; also the right-angled triangle BCD, with BCD equal 

 to the deviation ; lastly the right-angled triangle C D E, in which 

 CE\& equal to A C. 



Then G D B! is the required apparent angle. 



Proof. — If ^ ^ C^ be a vertical plane along the full dip, and GDE 

 the vertical plane of section. BCD will be a horizontal plane, and 

 AG, CE will coincide, so that BD, AE will be the plane of stratifi- 

 cation, giving the apparent angle G JD E along the section . 



Problem 2. — From two apparent dips ^ to find the full dip and its 

 direction. 



' Any three points, not being in a straight line, on a line of out- crop, will furnish 



