THE TRIASSIC PERIOD. 



11 



The means of distinguishing extrusive lava sheets from sills are 

 various, though all criteria are not usually, applicable in any one spot. 

 Some of these criteria are as follows: (1) The upper surface of an ex- 

 trusive sheet is likely to be more or less scoriaceous; (2) the basal 

 portion of the sedimentary rock overlying an extrusive sheet is likely 

 to contain fragments derived from the igneous rock beneath; (3) the 

 base of the clastic bed above an extrusive sheet has not been baked 

 by the heat of the underlying lava. In the case of the intrusive lava 

 sheets, or sills, on the other hand, (1) the overlying sedimentary beds, 

 as well as those below, have been affected by heat; (2) the upper 

 portion of the lava is not likely to be notably scoriaceous ; (3) the over- 

 lying clastic beds do not contain fragments of the igneous rock; (4) 

 the upper part of the igneous rock may contain fragments of the over- 

 lying sedimentary rock; (5) intrusive sheets are likely to send off 

 small dikes or stringers of lava which cut through few or many of the 

 layers of the overlying sedimentary rock; and (6) the intrusive sheet 



Fig. 314. — Diagram showing a sheet of intrusive rock (sill). 



itself may cross layers. Fig. 314 shows some of the characteristics 

 of an intrusive sheet, or sill. 



Structure. — The structure of the Newark series is generally mono- 

 clinical. In the Connecticut River valley 1 the dip is 10° to 25° (usu- 

 ally 20° to 23°) to the eastward (Fig. 316). The strata are other- 

 wise somewhat deformed, though never closely folded. In addition 



1 For an excellent summary of the Trias of Connecticut, see Davis, 7th Ann. Rept 

 U. S. Geol. Surv. A fuller and later account is given in the 18th Ann. Rept., Pt. II. 

 See also Emerson, the Holyoke folio, U. S. Geol. Surv., and for the interesting Pom- 

 peraug area, Hobbs, 21st Ann. Rept. U. S. Geol. Surv., Pt. III. 



