FENNER, THE WATCHVNG BASALT HI 



grate. In the more open channels of these rocks, the circulation was 

 naturally much more rapid than in the capillary spaces. Some difference 

 in concentration of dissolved material thus arises, and to this cause in 

 part is probably to be attributed a marked banding parallel with cracks, 

 which appears in many slides (Plate XII, figs. 3 and 4). 



It appears from what has preceded that the glassy crusts of the 

 bowlder-like forms of basalt, lying next to relatively open channels and 

 being in a chemically metastable condition, were naturally the portions of 

 the rock most attacked. Alteration also follows the shrinkage cracks 

 which penetrate the normally crystallized interior of the bowlders and 

 produces veinlike bands of secondary minerals in a breccia of unaltered 

 basalt. There is also evidence that deformative stresses acted upon the 

 region within a comparatively short period of the extrusion of the basalt, 

 for shear-planes often show alteration. On the whole, however, move- 

 ments or mechanical disturbances of any nature appear to have been 

 almost negligible during recrystallization. 



EVIDENCES AS TO TIME INVOLVED 



It has not been found possible to determine from field observations 

 what depth the flow of lava reached. At the quarry at West Paterson 

 which has been mentioned, a section shows a thickness of 60-70 feet from 

 the underlying shale to the surface of the ground, throughout which the 

 characteristic features of pahoehoe are developed. This merely indicates 

 what the minimum thickness was at this point and does not afford infor- 

 mation as to the amount removed by erosion. There are indications, 

 however, that the entire thickness of the First Watchung sheet was not 

 involved, for in several localities, it has been found that the phase of the 

 basalt in which the bowlder-like structure is prominent passes under 

 normal massive basalt, which appears to be a later flow. If this inference 

 is correct, the whole history of alteration must have occupied only a com- 

 paratively brief period, geologically considered ; for a second flow of lava, 

 it would seem, would spread an impervious seal over the entire area and 

 terminate the process at whatever stage had been reached. From other 

 considerations also, the history is believed to have been brief. A sheet of 

 lava, even several hundred feet in thickness, subjected to the cooling 

 effects of percolating waters^ would not retain its store of heat for a 

 period which would be considered long from a geologic standpoint. 



DEGREE OF SUPERHEATING OF THE SOLUTIONS 



Although a rapid circulation has been spoken of in referring to the 

 movements of the waters, this term has been intended merely in a relative 



