36 H. W. Fairbanks — Contact Metamorphism. 



Art. Y . — An Interesting Case of Contact Metamorphism / 

 by H. W. Fairbanks. 



Black Mountain is the highest peak of the El Paso range, a 

 spur of the Sierra Nevada mountains extending easterly into 

 the Mojave desert. The mountain owes its name to the mantle 

 of dark lavas which covers it. The underlying rocks consti- 

 tute a part of an extensive series of sedimentary beds exposed 

 for many miles along the northern slope of the El Paso range. 

 They consist of sandstones, clays and conglomerates and con- 

 tain in places much fragmental volcanic material as well as 

 occasionally interstratified flows. Last Chance gulch and its 

 tributaries drain the western slope of Black Mountain and in 

 the canons the character of the sedimentary beds, as well as 

 their relation to the volcanic flows, is often finely shown. The 

 sedimentary beds here have a light yellowish or pinkish color 

 and exhibit in places a finely banded appearance. In the field 

 they were thought to be wholly of volcanic origin, but a micro- 

 scopic examination revealed the fact that the light-colored 

 paste in which the more distinct fragments were embedded 

 consists of an amorphous kaolin-like substance. The small 

 partly-rounded pebbles and grains appear to be of many kinds, 

 but those of a volcanic nature predominate. 



The strata have been considerably disturbed and faulted, and 

 in one of the canons have been intruded by two dikes. One 

 of these has a diameter of less than one foot while the other is 

 14 feet across. The larger one cuts vertically through the 

 sedimentary rocks which dip at an angle of about 25 degrees. 

 This dike appears very fine-grained, but a microscopic examina- 

 tion shows that it is noncrystalline. The feldspar is probably 

 labradorite and occurs in long laths. The augite has a pale 

 brown color and gives an ophitic structure to the rock. 

 Abundant grains of a reddish color and presenting the appear- 

 ance of having resulted from the alteration of olivine are scat- 

 tered through the rock. Owing to the absence of a glassy 

 base the rock is then an olivine diabase. The surface of the 

 outcrop is quite decomposed and weathers away as rapidly as 

 the soft and slightly tufaceous beds. 



The remarkable feature connected with the intrusion is the 

 striking manner in which the adjoining rock has been meta- 

 morphosed. The thickness of the band of altered tufa is about 

 two feet where it is best exposed. The light colored-soft rock 

 has been baked to a dark hard and very firm one, the slabs of 

 which give forth a ringing sound when struck. A microscopic 

 examination does not reveal any new minerals, but only the 

 fact that the alteration has brought out more clearly the vol- 



