FENNER, THE WATCHVNO BASALT 185 



a younger formation in secondary vugs in a hydronephelite spreustein formed 

 from elfpolite. 



Younger than the natrolite is apophyllite in all occurrences where it ap- 

 pears ; it is, however, in comparison with the soda-rich zeolites — analcite and 

 natrolite — almost always present in wholly subordinate amount. I have not 

 observed it with stilbite; it is, therefore, not possible to say with certainty 

 whether the stilbite or the apophyllite is the older of the two ; but to judge by 

 the character of its occurrence, the Inst may be considered with some proba- 

 bility as almost always the youngest of all zeolites of our veins. Apophyllite 

 also belongs to the zeolites of Plombieres made known by Daubr^e, formed at 

 a late period at only about 70°. 



4. Phase of fluo-carbonates and carbonates, etc. 



... As the youngest formation in the vugs, perhaps deposited at almost 

 ordinary temperature, one finds here and there calcite, usually only compact, 

 seldom in idiomorphic crystals — for example, upon natrolite of Riso ; only 

 traces of a chloritic ( ?) mineral have I observed as a still younger deposit upon 

 the calcite. 



E:^suME 



The zeolites and associated secondary minerals of the Watchung basalt 

 are found in certain localized areas of the sheet. These areas possess 

 well-marked peculiarities of structure which are plainly of primary 

 origin and due to conditions present at the time of consolidation. The 

 chief peculiarity in these limited areas is an accumulation of bowlder- 

 like masses of "pahoehoe" lava, which consist of an interior of normally 

 crystallized basalt and an exterior crust of glass. From a study of the 

 geological relations in the field, it is believed that the basalt sheet repre- 

 sents a surface flow of lava poured out over a continental area which had 

 been depressed by crustal movements of deformation (either warping or 

 faulting) and in whose lowerlying portions a lake or series of lakes occu- 

 pied shallow basins. The pahoehoe structure of the basalt is believed 

 to have been developed over or immediately adjacent to the lake beds 

 through quicker cooling of the flow. 



The secondary minerals are held to have been developed from the 

 elements of the basalt (especially the glassy crusts) and from sublimates 

 given off by the magma in cooling, and deposited in crevices of the flow. 

 The medium in which recrystallization took place was an aqueous solu- 

 tion, which was enabled to percolate through the sheet of basalt, because 

 of the more permeable structure above the lake beds. A purely meteoric 

 origin is indicated for the waters, which probably worked in from the 

 edge of the sheet through the porous sandstones below and returned to 



