AHT. 2. PETROLOGY AT GOOSE CEEEK SHANNON. 21 



Similiar coarsening in spots is a common phase of igneous rocks 

 and has many times been noted in diabases and gabbros. Regard- 

 ing the gabbro of the Baltimore area Williams states:^ 



The most striking feature in the texture of the unaltered gabbro is tlie re- 

 peated and abrupt change in the coarseness of grain which is seen at some local- 

 ities. This phenomenon, as is well known, is one frequently observed in very 

 ancient massive rocks which cover considerable areas. It was undoubtedly 

 ■caused by some irregularity in the cooling of the original magma from the 

 anolten state, for which it is now difficult to find a satisfactory explanation. 



Coarse phases of the Duluth gabbro have been described by 

 Orout ^ as follows : 



There are patches in the banded gabbro, especially near the base and near 

 the top, in which the gabbro minerals have grown coarse, with grains up to 6 

 inches in diameter, and since the borders are ill-defined the masses may be at- 

 tributed to processes of segregation. Miarolitic cavities and a little biotite may 

 be taken as indications of the presence of mineralizers but the biotite is scarcely 

 more abundant than in some common bands of the gabbro. The patches of 

 notably coarse grain range from a few inches to many feet across and are esti- 

 raated from incomplete exposures to be roughly ellipsoidal to somewhat tabular 

 in form. In many places near the base the patches are numerous. 



Numerous other specific occurrences could be quoted but the pre- 

 ceding serve to show that there is nothing unique about the coarse 

 diabasic pegmatites of Goose Creek. The discussion of Iddings* is 

 concise and pertinent: 



Another case of heterogeneous texture is found in rocks often of intermediate 

 composition, but also in others, in which in certain spots all the mineral com- 

 ponents appear in relatively large crystals compared with those in surrounding 

 portions of the rock. Apparently at these spots conditions existed favorable to 

 the formation of large crystals. These were most likely molecular mobility of 

 the magma, probably produced by a slightly greater content of gas, for a small 

 amount that would initiate crystallization would remain in the liquid since it 

 does not enter into the composition of the crystallizing solids. 



Heterogeneous texture is characteristic of most pegmatitic rocks, especially 

 those composed of feldspar and quartz. In them coarsely graphic fabric and 

 radial fabric commonly mingle with granular consertal fabric, which may be 

 equigranular in some places and inequigranular in others, often varying greatly 

 in granularity. 



There is nothing in the foregoing abstracts which is inconsistent 

 with the conclusion that the diabase pegmatite, as described above, 

 owes its texture to segregation of mineralizers into spots yielding a 

 fluid gas-rich magma which remained fluid after the solidification of 

 the surrounding diabase and permitted the growth of large crystals. 



•George H. Williams, The gabbros and associated hornblende rocks occurring in the neighborhood of 

 Baltimore, Md. U. S. Qeol. Survey, Bull. 28, p. 25, 1886. 

 'Frank F. Grout, The pegmatites of the Duluth Gabbro. Econ. Oeol., vol. 13, p. 185, 1918. 

 'Joseph P. Iddings. Igneous Rocks, vol. 1, p. 242, 1909. 



