412 JR. D. Oldham— Notes on a Graphic Table of Dips. 



indubitably intrusive, exbibit a structure close]}'- allied to tbat of 

 peridotites and identical with tbat of other serpentines, where the 

 evidence is inconclusive. Do we as yet know of any case where 

 a well-marked and definite general structure is common both to 

 a rock of sedimentary and to one of igneous origin? Is it not a 

 legitimate conclusion, that serpentine is of a common origin with 

 peridotite, and that the latter is one of the igneous rocks ? 



Hence I think I am justified in saying that, notwitlistanding the 

 ingenuity of Dr. Hunt's reasoning and his skilful special pleading, 

 no theory regarding the origin of peridotites and serpentines can be 

 held to be complete which does not take account of the fact that 

 some^ of them are as fully proved to be intrusive rocks as any 

 dolerite or basalt, felstone or trachyte. 



V. — Note on a Graphic Table of Dips. 

 By R. D. Oldham, A.R.S.M. 



THAT there is a wide-felt want, among field-geologists, of some 

 rapid and sufficiently accurate method of solving such problems 

 as arise in the ordinary course of their work is proved, if proof were 

 necessar3^ by the summary of the literature bearing on this subject, 

 given in the April Number of this Journal by Mr. Harker, who has 

 certainly carried it to its highest point, for methods more simple 

 than those given by him it is impossible to imagine. There is, 

 however, a method, by which most of these problems can be solved 

 by inspection without recourse to construction or calculation, which 

 I would submit to the notice of geologists in general. 



The problems arising in the ordinary course of field geology are 

 mainly of four kinds : (1) where the true dip is known and the dip 

 in the direction of a line of section is required ; (2) where two ap- 

 parent dips are known and the true dip is required ; (3) problems 

 connected with the outcrop of beds on sloping ground ; and (4) those 

 connected with the tilting of beds already inclined. I omit the 

 obtaining of the thickness of a series of beds whose dip and breadth 

 of outcrop are known, as the solution is self-evident to all who can 

 understand what is meant by 'scale'; of these, class (1) is of by 

 far the most frequent occurrence, while cases falling under heading 

 (4) are somewhat rare. I propose describing ni}'^ method of con- 

 structing what may be called a graphic table of dips by which all 

 cases falling under heading (1) can be solved by inspection, and then 

 pasfiing on to an extension of the principle by which cases falling 

 under headings (2) and (3) may be similarly solved. 



Draw a square as OABG (Fig. 1), of convenient size, and with 

 centre and radius OA strike a quadrant inside the square ; divide 

 this quadrant into equiangular distances of convenient magnitude, 

 and draw radii at those intervals from 0, prolonging them till they 

 cut the opposite sides of the square ; from the points where the radii 

 cut the side AB, perpendiculars are to be drawn to the base (W, and 

 with centre and radii equal to the distances so cut off a concentric 



