826 PRINCIPLES OF STRATIGRAPHY 



series with that of the first stratum unconformably overlying the 

 series. 



Unconformities and disconformities are often suggested by the 

 occurrence of dikes of igneous material in the lower rocks, which 

 do not penetrate the upper beds. Stronger folding or faulting in the 

 lower than in the upper series also suggests an unconformity, as 

 shown below. As a unique example of an indicated disconformity 

 may be cited the sandstone dikes filling fissures in the Siluric lime- 

 stones of western New York and Canada, the overlying beds being 

 wholly unafifected by these fissures and dikes because a later de- 

 posit. Faulting affecting a lower set of strata, but not a higher one, 

 also indicates a disconformable relationship, and a hiatus represent- 

 ing a sufficient time interval to allow for the removal of the fault 

 scarps of the low^er series. 



The appearance of an inverted unconformity, where the later 

 strata end abruptly against the older ones, may be produced by in- 

 tense folding of the rocks of a complex region, as shown in the 

 figures of the structure of the Aletschhorn on page 825. (Figs. 203- 

 204.) 



Conformable or accordant strata may also show a variety of as- 

 pects. Crosby has shown that on the Atlantic coastal plain of North 

 America, the strata wedge out landward.* This wedging conformity, 

 where the formations are thinner in one locality than in the other, 

 though fully represented, Crosby has called sphenoconformity, and 

 its correlative, where the strata are of uniform thickness, he calls 

 planoconformity. If contemporaneous faulting of the older series 

 goes on with the deposition of the newer, fractoconformity is pro- 

 duced. 



BIBLIOGRAPHY XX. 



1. BROWN, THOMAS C. 1913. Notes on the Origin of Certain Palaeozoic 



Sediments, Illustrated by the Cambrian and Ordovician Rocks of Center 

 County, Pennsylvania. Journal of Geology, Vol. XXI, pp. 232-250. 



2. CROSBY, WILLIAM O. 1893. The Origin of Parallel and Intersecting 



Joints, American Geologist, Vol. XII, pp. 368-375. 

 2a. CROSBY, W. O. 1912. Dynamic relation and terminology of strati- 

 graphic conformity and unconformity. Journal of Geology, Vol. XX, 

 No. 4, 1912, pp. 289-299. 



3. DAUBREE, AUGUSTE. 1879. Etudes synthetiques de Geologie Ex- 



perimentale, pp. 300-374. 



4. DAVIS, WILLIAM M. 1913. Nomenclature of Surface Forms on Faulted 



Structures. Bulletin of the Geological Society of America, Vol. XXIV, 

 pp. 187-216. 



* A part of this is no doubt due to actual breaks in the series, which disappear 

 seaward. 



