326 



PHYSICAL GEOLOGY 



Dikes are found cutting rocks of all ages, and they extend across 

 the country without reference to topography. 



Sills. — Lavas which have been forced between sedimentary strata 

 and have formed sheets which have a small thickness as compared 



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r^^-flT 



==7-- 



■rr^T~~-^- 



^tr^F 



^py^-^^^. 



Fig. 322. — Diagram illustrating the relation of the Palisades of the Hudson 

 (vertical lines) to the strata in which this sill was intruded. 



with their extent are called sills (Fig. 322). Sills sometimes extend 

 long distances along the same bedding plane, but often cut across 

 from one stratum to another. They vary in thickness from a few 

 feet to several hundred feet and sometimes have an extent of many 

 square miles. When they have been exposed by erosion, they can 

 be distinguished from extruded lavas by the absence of vesicular 



Fig. 323. — The Palisades of the Hudson. The sheer face of the upper portion is 

 due to the vertical jointing of the trap, and to the more rapid erosion of the weaker, 

 underlying rock. (Photo. D. W. Johnson.) 



lava on the upper surface. The Palisades of the Hudson, which ex- 

 tend for 30 miles along the west bank of the river as a bold cliff several 

 hundred feet high, form a part of a sheet of intrusive lava (sill) which 

 is underlain by sandstone and was formerly overlain by other sedi- 

 mentary strata (Fig. 323). 



