. Bulletin of the Geological Society of America. Geology. CHARACTER OF GABBRO-LIMESTONE CONTACTS. 275 close together, although the line of junction was not exposed. These observations show that the contact is extremely irregular, presenting all the characteristic features produced by the intrusion of an igneous rock. In the vicinity of Natural Bridge the contact is shown on a horizontal surface, forming a very irregular line, the gabbro sending out broad ex- tensions into the limestone. Sometimes the latter rock cuts off portions of the gabbro from the main body, forming isolated patches. Narr
. Bulletin of the Geological Society of America. Geology. CHARACTER OF GABBRO-LIMESTONE CONTACTS. 275 close together, although the line of junction was not exposed. These observations show that the contact is extremely irregular, presenting all the characteristic features produced by the intrusion of an igneous rock. In the vicinity of Natural Bridge the contact is shown on a horizontal surface, forming a very irregular line, the gabbro sending out broad ex- tensions into the limestone. Sometimes the latter rock cuts off portions of the gabbro from the main body, forming isolated patches. Narrow, sharply defined dikes of the gabbro have not been observed, the exten- sions being broad and irregular. These contacts, however, though evidently irruptive, are not as striking as those presented in vertical sections. A most favorable locality for observing one of the latter is /S in a cliff on the west shore of Bonaparte lake. It is prob- able that this is the locality referred to by Van Hise* in his brief account of the region, as it corresponds closely to a description of the latter point given to the writer by the late Professor G. H. Williams shortly be- fore his death. The gabbro here exposed may not be continuous on the surface with the main body ; but if not, there is no doubt of their unity, so that the bearing of the facts remains the same in either event. The cliff referred to rises almost vertically out of the water to a height of sixty or seventy feet. Near its base the limestone is exposed clearly banded, but the mass of the cliff consists of gabbro, which cuts across the banding of the limestone in a sinuous line (see figure 4). At one point a wedge of limestone projects into the gabbro a distance of three feet. The limestone extends to the top of the cliff, but is cut off again by the gabbro a short distance along the face. These phenomena are repeated several times at this point and elsewhere along the lake shore, the gabbro cutting through the limestone ag
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