302 THOMAS L. WATSON 



Quartz is present in very subordinate amount in many of the 

 sections, and it is probably largely, if not entirely, secondary. Mus- 

 covite and calcite are wholly secondary, and as such they present 

 no noteworthy features. Magnetite and an occasional zircon com- 

 plete the hst of minerals. 



Summing up the results of the microscopic study, we find that 

 the sections consist essentially of diallage, uralitic hornblende, and 

 a basic plagioclase, showing, as a rule, but shght polysynthetic 

 twinning, and usually altered to muscovite and calcite, or to zoisite 

 and muscovite. The presence of perfectly fresh microcline in sub- 

 ordinate amount, a httle quartz which is probably secondary, and a 

 relatively large amount of accessory titanite and apatite is charac- 

 teristic. Clearly the rock is a gabbro-diorite, although the presence 

 in some of the sections of microchne and quartz is unusual. 



ORIGIN OF THE NODULAR STRUCTURE. 



Backstrom accounts for the origin of certain European nodular 

 granite on the basis of magmatic differentiation, in which the nodules, 

 hke the Carohna occurrence, are more basic than the matrix.^ This 

 seems to be the most satisfactory explanation of the Carolina occur- 

 rence, but whether the beginning of the nodules resuhed from greater 

 basicity, differential coohng, or from some other cause, it is not pos- 

 sible to state definitely. 



From the relations of the minerals in the rock it appears that the 

 dark sihcates generally preceded the feldspar in crystalhzation, 

 although overlapping in the periods of separation from the magma 

 of these minerals is distinctly shown. The primary accessories, 

 apatite, titanite, iron ore, etc., are idiomorphic in outline and were 

 the earliest minerals to crystallize from the magma. Microscopic 

 evidence further indicates that the nodules were developed in the 

 magma when crystallization was fairly well advanced. 



RESUME. 



Briefly summarized, the principal results of this study are: (i) 

 The rock is a gabbro-diorite whose field relations strongly suggest 

 occurrence in the form of a dike penetrating a gray porphyritic biotite 



I Journal of Geology, Vol. I (1893), pp. 773-90; Ceologiska Fdreningen i 

 Stockholm, Fdrliandlingar, 1894, p. 128. 



