. The Crystal Falls iron-bearing district of Michigan. Iron ores; Geology; Geology, Stratigraphic; Geology, Stratigraphic. ACID DIKES IN ARCIIEAN. 45 ACID DIKES IN ARCHEAN. Observations upon diki's of acid rocks {•uttini^- the Arcliean <»-r;uiite are very i'ew, and we may suppose this to be partly due to tlieir occurrence in isolated knobs, which prevented the determination of the relations of adjacent exposures of rocks of sliji'litly different character. Some few dikes were, nevertiieless, ol)serve<l, and are j^Tauites varying from medium to coarse gi-ained, grauolitic' to })orphyritic
. The Crystal Falls iron-bearing district of Michigan. Iron ores; Geology; Geology, Stratigraphic; Geology, Stratigraphic. ACID DIKES IN ARCIIEAN. 45 ACID DIKES IN ARCHEAN. Observations upon diki's of acid rocks {•uttini^- the Arcliean <»-r;uiite are very i'ew, and we may suppose this to be partly due to tlieir occurrence in isolated knobs, which prevented the determination of the relations of adjacent exposures of rocks of sliji'litly different character. Some few dikes were, nevertiieless, ol)serve<l, and are j^Tauites varying from medium to coarse gi-ained, grauolitic' to })orphyritic rocks. The porphyritic facies is the most conuuon. The dikes do not show differences from the main mass of the Archean granite suflKcieiit to warrant detailed petrographical description. The following description of one mass of granite-porphyry is given, as it offers good proof of its relation to the schistose border facies of the granite. In this case the gneissoid rock is found as inclusions in the granite-porphyry, as is illustrated in the accompanying diagrammatical sketch, fig. 4, taken from a ledge in the field. In this sketch the sharply outlined angular and lenticular areas represent the gneiss included in the gi'anite-porph)Ty. The jjlienocrysts of this granite-porphyry have a parallel arrangement, the long direction of the phenOCryStS agreeing also with the trend Fio 4—Granite with mclusions of gneis- soid granite. of the longer axes of the inclusions. The banding and foliation in the inclusions strike at a right angle to the flow- age structure of the granite. The lines of separation between the areas of gneiss and the granite, as shown in this outcrop, are sharp, and |)oint to their nature as inclusions, and such is accepted as the true explanation of their angular character and sharp outlines. As this por^jhyritic granite was intruded throu"h the border facies of the Archean oranite, these fras"- ments were taken up, and were so arranged a
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