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 tlie 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 gabV)ro from the main body, forming isolated ])atches. Narrow, 

 shar])ly 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 

 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 

 clot^ely 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 bodv 

 not, there is no doul 

 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 ])oint 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 tlie gabbro a short distance along the face. These 

 phenomena are repeated several times at this i)oint and elsewhere along 

 the lake shore, the gabbro cutting through the limestone again and 



y ; but if 5fS^|5^;^|*^^ 



)t of their ^„^K'd/^^^ 



FiouBE 4.—Oabbro-Lime8tone Contact. Lake Bonaparte. 

 The drawing is made from a photograph. 



•Bulletin 8G, U. S. (iftol. Survey, p. 399. 



