VARIATIONS OF TEXTURE IN CERTAIN TERTIARY 

 IGNEOUS ROCKS OF THE GREAT BASIN. ^ 



INTRODUCTION. 



During the field work of the writer in the Great Basin, in 

 1899, he found in a number of localities, chiefly near the edge 

 of the Sierras, cases of what seemed at first extraordinary transi- 

 tions or intimate alternations of texture in the Tertiary igneous 

 rocks, which are for the most part extrusive. Microscopic and 

 comparative examination in the office corroborated the field 

 conclusions and showed also that the transitions were not as 

 abrupt as they seemed at first, but were more gradual ; and the 

 study of the conditions of crystallization which may be inferred 

 from the structure of the different varieties shows that similar 

 conditions must exist in many other places and similar transi- 

 tions may be looked for. It is true that there are in that por- 

 tion of the Great Basin especially under consideration (namely, 

 the district lying within fifty miles of Carson) exceptionally 

 favorable circumstances for the exposure of both the surface 

 portions and the originally deeply buried portions of lavas. 

 The region is an arid one, and therefore the general erosion is 

 slight ; nevertheless, waters derived from the moister Sierras 

 reach out into this region in the form of streams, and have 

 accomplished much special or basal ^ erosion. There are also a 

 number of lakes, which in former times were much more exten- 

 sive, and probably existed in one stage or another since early 

 Tertiar}^ times ; and the basal erosion of these lakes has proba 

 bly been considerable. 



Some of the special localities where the observations were 

 made will now be described in detail. 



' Published by permission of the director of the United States Geological Survey. 



^ 1. e.. Erosion which works at the base of topographic features, undercutting 

 them. 



