A. Wandke — Study of Cape Neddick Gabbro. 301 



relative amounts of light and dark constituents, any two 

 adjacent bands throughout the banded area being seem- 

 ingly like any other two adjacent bands and the two 

 selected bands differing perhaps but little in average 

 composition from the unbanded rock into which the 

 banded rock grades. Some process, acting intermit- 

 tently and rhythmically, for the bands are approxi- 

 mately of the same width in any random two-foot zone, 

 would best account for the field relationships. It seems 

 to the writer that the pulsatory escape of mineralizer may 

 best be appealed to as the mechanism responsible for the 

 banding. The steeply inclined thinly bedded sediments 

 would have furnished excellent avenues of egress to the 

 volatile components. These sediments, moreover, for 

 over one-hundred feet distant from the contact are not 

 only recrystallized but show evidence of the addition of 

 new constituents. The abundant apatite in the contact 

 phase as well as the marked alteration of pyroxene in 

 the banded phase to hornblende and biotite also indicate 

 the presence of mineralizers. The escape of the volatile 

 components would at once upset pressure, one of the 

 factors of temporary equilibrium. This upset in pres- 

 sure would manifest itself not in a local change in equili- 

 brium but in a change that would at once be transmitted 

 throughout the entire magma. If now the body be 

 thought of as crystallizing in the normal way, from the 

 margin inward, then the change in equilibrium might 

 make itself most apparent in this marginal zone, a banded 

 rock being the result of recurring upsets in equilibrium. 



Central phases. — The central portion of this stock is 

 occupied by two -phases showing as marked differences in 

 composition as do any two adjacent bands of the pre- 

 viously described banded phase of the gabbro. The 

 surface of this central portion, practically clear of debris, 

 could be studied in detail and the phases mapped without 

 exaggeration. As shown by the map the relations are 

 almost such as to suggest that these two contrasted 

 phases had differentiated and then the magma in the 

 stock been given a swirl with a gigantic mixing spoon. 

 The essential minerals of the dark phase are olivine, 

 magnetite, pyroxene, pyrrhotite, hornblende, biotite and 

 labradorite. The hornblende and biotite both replace the 

 pyroxene. The biotite in addition forms about and 



Am. Jour. Sci. — Fifth Series, Vol. IV, No. 22. — October, 1922. 

 20 



