Correspondence — Professor Preshvich. 



237 



Note.— These rules will not apply unaltered to lines of ap- 

 parent dip enclosing acute angles, which in practice are rarely 

 found. In such cases a horizontal triangle constructed by the 

 reverse rule, i.e. on the other side of either line of observed dip, 

 will give the result required. 



EiG. 3. 



The quarry represented in Fig. 3 may be taken as an example of 

 obtaining the true dip by these methods. The operation being 

 repeated under varying conditions with the same result. 



H. M. Geological Survey. W. H. Penning. 



THICKNESS OF THE OXFOED CLAY. 



Sir, — As few of your readers may possibly see my Lecture on the 

 Water Supply to Houses and Towns with especial reference to 

 Oxford, I beg to enclose an extract from' it, having reference to a 

 question which is of some geological interest, viz. the thickness of 

 the Oxford clay in this district. The inquiries I had occasion to 

 make respecting the practibility of an artesian well at Oxford led 

 me to doubt previous conclusions. In the following extract frpni 

 my pamphlet I give the geological grounds on which I arrived at this 

 different result. 



" We know generally that the Oolites become thinner in their range 

 eastward, but the Oxford clay has, on the contrary, been thought to 

 become thicker, and it has been generally stated, on the evidence 

 chiefly of the boring made at Wytham ^ in 1829, that at Oxford it is 

 600 ft. thick, or more. 



"I am satisfied however, that there is some error in the construction 

 put on that section, and that the thickness of the Oxford clay here is 

 much less than that estimate. A record of the boring has been pre- 

 served, which shows it to have been carried through a succession of 

 1 Three miles N.N.W. from Oxford. 



