312 C. N. FENNER 



word pahoehoe is formed by small offshoots of very hot and highly liquid lava 

 from the main stream, driven out laterally or in advance of it in a succession of 

 small belches. These spread out very thin, cool quickly, and attain a stable form 

 before they are covered by succeeding belches of the same sort.^ 



Above the reticulated strip described comes fifteen feet of nearly 

 massive trap with few joints or planes of separation, and above this, 

 along an irregular line, there is a blocky variety of considerable thick- 

 ness. Except for the narrow band of vesicular trap on the contact 

 the texture of the whole mass is firm and dense and the rock is of flint- 

 like hardness. 



We may continue southward for several miles along the cliffs mark- 

 ing the eastern face of the mountain, and find almost identical condi- 

 tions everywhere. The base of the trap sheet shows slight irregulari- 

 ties but is practically conformable with the bedding of the sandstones 

 and shales. For a short distance above it is apt to be somewhat 

 porous, but the great upper mass is perfectly dense. 



Evidence of the existence of a lake bed, covered by the lava flow. — 

 Let us now follow the D. L. & W. R. R. westward from Garret Rock 

 to the West Paterson trap quarry. Garret Rock is a bold cliff mark- 

 ing the eastward scarp of the mountain, and from here the railroad 

 makes a section almost at right angles to the line of the ridge. About 

 three thousand feet from the rock it passes the quarry mentioned, 

 which is in the very midst of the trap area. The quarry has been 

 famous to mineralogists for years for the specimens of zeolites and 

 associated minerals which have been obtained from it. It is only 

 within about a year, however, that operations have proceeded so far 

 that the geological conditions have become plain and the whole story 

 revealed so that it can be easily read. The section is so perfect, and 

 the rock and its minerals are so fresh and unaltered that a careful 

 study of the exposure should be made. We shall find here the expla- 

 nation of features occurring elsewhere, which by themselves might 

 convey little meaning. 



The first observation which will probably be made is that here in 

 the midst of the trap area the bottom of the quarry shows a floor of 

 hardened red mud rising from the west toward the east. On this 

 the trap rests. Along the contact for a width of ten feet or more the 



I Fourth Annual Report, U. S. Geological Survey, pp. 95, 96. 



