ABT. 2. PETROLOGY AT GOOSE CREEK SHANNON. 33 



If we inquire into the conditions of the formation of adinole from slates, we 

 will find that wholesale introduction of albite as such is not necessary. Some 

 magnesia, iron, and alumina are lost by the sediment. Silica has probably not 

 been introduced, for the loss of the above-mentioned constituents suffices to in- 

 crease the silica to the percentage in adinole. Finally potash, too, is lost and 

 at the same time is replaced by soda. Carbonate waters, bearing a little soda, 

 could accomplish the work necessary. That such waters exist in basaltic mag- 

 mas and have important effects during the late stages of crystalHzation is the 

 conclusion of Bailey and Grabham in a late article. If the conclusions of the 

 present writer are correct, such waters, emanating from the diabase, have pro- 

 duced the adinole and the adinole-rich granophyre here described. The waters 

 supplied most of the soda and the sediment supplied alumina and silica. 

 Calcite is an almost universal constituent of the aplite veins associated with the 

 granophyres. It has in some cases apparently crystallized together with the aplite 

 minerals. This certainly points to the presence of carbonated waters. * * * 

 That the granophyre "solution," formed as here imagined, was foreign to the 

 diabase magma is indicated by the intense alteration of the constituents of the 

 diabase near the granophyric interstices. 



The aplitic veins (quartz and albite, often with calcite) which cut both gran- 

 ophyre and diabase, were formed from the more acid residuum of the granophyre. 

 They are especially numerous near a mass of granophyre. The e.xtreme purity 

 of the albite * * * points to their aqueous origin, as does also their calcite 

 content. This aqueous residuum probably deposited also the valuable metallic 

 content of the aplite veins and of the associated calcite veins. 



Another much studied example of the association of very basic 

 gabbroic rocks, with acid differentiates contrasting markedly in com- 

 position, is the Duluth gabbro mass with its associated "red rocks," 

 which are largely composed of quartz-feldspar micropegmatite. The 

 gabbro itself has formed numerous varieties by some modification of 

 the process of gravitative differentiation. The red rocks have been 

 carefully described by Grout, ^"^ from whose paper the following ab- 

 stract is taken: 



The gray gabbro rapidly gives place to a bright red rock very different from 

 the gabbro in mineral, chemical, and physical characters. * * * 



The chief outcrops near Duluth are irregular patches at the top of the main 

 gabbro and apophyses into its roof; it occurs also near the top of the earlier feld- 

 spathic gabbro, in a large sill close above the gabbro, and in some small dikes 

 near the bottom of the gabbro. 



The texture varies from sugary near contacts to very coarse in certain patches. 

 The rock is pecuharly friable, so that hand specimens can hardly be trimmed 

 from it. A striking local variation contains long needles of dark minerals in a 

 red matrix. In thin sections it is micropegmatitic, varying to granitoid in some 

 large masses. Miarolitic cavities are numerous in some places. Variability is 

 as characteristic of the minerals as of the textures. The chief red mineral is a 

 feldspar stained with considerable hematite and badly kaolinized. Probably 

 most red rock contains two feldspars; zoning is especially common in the phases 

 grading into the gabbro. Quartz, though abundant, is rarely visible except with 

 the microscope as an intergrowth. Hornblende is the chief ferromagnesian 



i« Frank F. Grout. A type of igneous differentiation. Jouru. Qeol., vol. 26, pp. 632-634, 1918. 

 94110—24 3 



