REVIEW OF EARLIER INVESTIGATIONS 443 



WORK OP A. R. LEDOUX 



The Hills were visited in the summer of 1890 by Dr. A. E. Ledoux 

 with the purpose of examining some iron ore prospects on East Butte. 

 At the meeting of the New York Academy of Sciences, February 9, 1891, 

 Dr. Ledoux gave an interesting description of East Butte. Approaching 

 one of its summits called Mount Morris, he records '.^ 



"The ground is thickly strewn with every kind of igneous rock — dark green 

 basalts, phonolite, and every variety of feldspar porphyry. . . . The moun- 

 tain mass ... is a grayish porphyry containing small feldspar crystals, 

 but gashed and seamed by dike after dike of trap and large crystal bird's-eye 

 porphyry. Between these dikes are older clay slates, mica schists, hornblendic 

 schists, all inclined at a sharp angle and indicating clearly the intrusive nature 

 of the mass.'' 



The iron ore lay at the contact of porphyry and marble. Dr. Ledoux 

 was thus impressed with the variety of the rocks present, and while he 

 mentions phonolite without microscopic determinations, he was entirely 

 correct. He also appreciated the contact effects. 



General Petrography 



As stated above, the earliest microscopic determinations of the rocks 

 were made by Frank D. Adams for George M. Dawson in 1882-84. Re- 

 ferring to East Butte, Dr. Dawson speaks as follows : . 



"The rock is very uniform, lithologically, in appearance and composition. 

 Mr. F. D. Adams has examined microscopic sections of it, and states that it 

 may be called a hornblende-trachyte, rich in plagioclase. Mr. Adams writes : 

 'It is composed of orthoclase and plagioclase, both present in large amount, 

 and some hornblende. It is therefore, intermediate in composition between 

 andesite and trachyte, and to which class it may be best referred can only be 

 ascertained by a partial analysis.' " ° 



This general determination by Dr. Adams holds very well for the main 

 laccoliths. About ten years later, 1895, Dr. Dawson furnished W. H. 

 Weed and L. V. Pirsson two specimens from West Butte which showed 

 a weathered l)rown crust and some flowage structure in the arrano-ement 



mapped the geology, as shown on the three flgures — S. 4, and Ti. We added some addi- 

 tional details in May, and since then Mr. Fowler has kindly filled out several gaps, more 

 especially in East Butte. The rather extensive collections of igneous rocks have been 

 worked up microscopically and the manuscript has been prepared by the senior writer. 

 Over one hundred thin-sections have been cut and studied. The sedimentai'y strata have 

 been described by the junior writer. We gladly taJve this opportunity of expressing our 

 indebtedness to Mr. Fowler for many accurate observations and much enthusiastic help. 

 We are also greatly indebted to Dr. Henry S. Washington for three complete analyses of 

 the igneous rocks. 



« Trans. N. V. Acad, of Sci., vol. x. 1891, p. .59. 



"Canadian Geological Survey. Report of progress. lS,Si.>-l,SS4. p]). 40C. 47('. 188.5. 



