//. B. Medlicott — On Faults in Strata. 341 



I beg to propose for this fish the trivial name of orhicularis. 



I am indebted for this specimen to the kindness of W. H. Huddle- 

 stone, Esq., r.Gr.S., who obtained it during a late geological excursion 

 to Lyme Eegis. 



EXPLANATION OF PLATE X. 



Eig. L ^cJimodus orhicularis, Morris, fi'om the Lower Lias, Lyme Regis, Dorset. 



Original specimen in the collection of the author (two-thirds nat. size). 

 Fig. 2. Teeth of JEchinodus (magnified three times nat. size) . 

 Fig. 3. Teeth oi Bapedius (magnified three times nat. size). 

 Figs. 2 and 3. Drawn from specimens in the British Museum, 



11. — On Faults in Strata. 

 By Henry B. Medlicott, B.A., Geological Survey of India, 



A LITTLE time back there appeared in the Magazine, some short 

 papers on the subject of faults,^ and on the nature of the conditions 

 and the forces through which these important structural features may- 

 have been produced. The points I would now bring to notice are more 

 elementary ; they refer to the evidence for faults ; hence involving 

 the principal data upon which the higher discussion of the phenomena 

 must be based, and the same data very largely affect our attempted 

 restoration and history of bygone phases of the earth's surface. 

 Faults and flexures in stratified rocks are the leading features 

 through which we interi^ret the disturbances that have afiected the 

 earth's crust; and any looseness in determining their existence, 

 form and amount, must vitiate many of our inferences. No one but 

 an experimental field geologist can appreciate the difficulty of such 

 determinations, and understand how faults are particularly liable to 

 elude observation. This circumstance accounts for, but does not 

 justify, the arbitrary use of faults in interpreting sections. To call 

 in question the evidence upon such a familiar subject implies, of 

 course, dissatisfaction at the manner in which it is handled in 

 practice. This I at once admit, and will proceed to explain. The 

 criticism I have to make is no more than might occur to one who 

 had never left his study ; but I would state that with me it has had a 

 most practical origin : in the progress of the work of the Geological 

 Survey of India, several great boundary faults have been proposed in 

 connection with our main rock-series, and in some cases published 

 descriptions have been already given ; but both on the score of the in- 

 sufficiency of the evidence brought forward, and after personal 

 examination in the field, I am unable to admit that some of the 

 features in question can, without very implicit qualifications, be 

 brought within the received definitions of a fault. I believe that it 



ning together of the terminal vertebra^ and such a proportion of the dermoha^mals, 

 as leads to an equal-lobed caudal fin, which has been termed ' homocercal ; ' but as it 

 is only symmetrical in contour, and remains more or less imsymmetrical in its frame- 

 work, I term it ' homocercoid.' The ganoid fishes of the mesozoic periods manifest 

 seyeral interesting gradations of this transitional state fi'om the hetcro- to the true 

 homo-cereal form, each step being a permanent character of the extinct species pre- 

 senting it. — Comparative Anatomy and Physiology of Vertebrates, 1866, Vol. i., p. 253. 

 1 See Geol. Mag, 1868, Vol. V., pp. 205, 339, 341, etc. 



