154 A. Harker — Graphical Methods in Field Geology. 



of all the collections would place the subject on a more satisfactory 

 footing, and that ultimately the group, with all its modifications, 

 may be thoroughly diagnosed. 



EXPLANATION OF PLATE VI. 

 The Hamus Group. 



Fig. I. Aiaria imiearinata, s-p.n. Dogger, Blue Wyke. York Museum. Front 



view. 

 ,, 2, 2a. Alaria tmicarinn tn , sp.n. Dogger, Blue "Wyke. Bean Collection, 



British Museum. Back view and whorl enlarged. 

 ,, 3, 3«. Ahfia Fhillipsii, d^^Ovh. Dogger, Blue "Wyke. Leckenby Collection. 



Back view and whorl enlarged. 

 „ 4. Alaria PMllipsii. Millepore Eock, Cloughton, Leckenby Collection. 



Back view. 

 ,, 5. ^^an«! P/j«7/eji?su', spiuulose variety. Dogger (?), Peak. My Collection. 



Front view, enlarged twice. 

 ,, 6, 6a. Alaria pseudo-armata, sp.n. Dogger, Blue "Wyke. Leckenby Col- 

 lection. Back and front view. 

 [The Hamus-group extends thus far only.] 

 ,, 7, la. Alaria bispinnsa, Phil, (variety). Kelloway Rock, Scarborough. 



York Museum. Back view, and spire enlarged. 

 ,, 8, 8a. Alaria bispinnsa, var. elegans. Cornbrash, Scarborough. Leckenby 



Collection. Back view and spire enlarged. 

 ,, *9, 9a. Alaria bispinosa, var. pinguis. Kelloway Rock, Scarborough. 



Leckenby Collection. Back view and spire enlarged. 

 ,, 10, 10a. Alaria bispinosa, var. pinguis. Kelloway Rock, Scarborough. 



Leckenby Collection. Back view and spire enlarged. 

 ,, *11, 11a. Alaria trifida, Phil. Kelloway Rock, Scarborough. Leckenby 



Collection. Back view and whorl enlarged. 

 * The specimens thus marked occur on the same block of stone. 



{To be continued.) 



11. — Graphical Methods in Field-Geology. 



By A. Hauker, B.A., F.G.S., 



of St. John's College, and 

 Demonstrator in Petrology in the "Woodwardian Museum, Cambridge. 



Introduction. 



IN determining the actual position of strata from the appearances 

 presented by their exposed edges in natural and artificial 

 sections, certain mathematical problems are of constant occurrence. 

 To a field-geologist who is not content with rough guesses founded 

 on judgments by eye, the solution of these problems is a matter 

 of importance, and methods have accordingly been given for some of 

 those most frequently met with. These solutions take the form of 

 (1.) trigonometrical formulee, which can be applied only with the 

 aid of trigonometrical and logarithmic tables; (2.) tables specially 

 jDrepared from these formulas for use in the field; (3.) graphical 

 methods, requiring only a ruler, scale and protractor, which may be 

 conveniently combined in one instrument. Of the first kind is the 

 formula for deducing the true dip of strata from two apparent dips, 

 given in Green's Geology (p. 341, 1st ed.), etc. Among special 

 tables are those of Mr. Jukes for finding the apparent dip in any, 



