BUILDING AND OKNAMENTAL, STONES. 463 



(2) VOLCANIC FRAGMENTAL ROCKS. TUFFS. 

 (a) Definition, Origin, and Composition. 



Under the general name of tuff it is customary to include those fine- 

 grained fragmental rocks formed by the consolidation of volcanic de- 

 tritus, such as ashes, sand, and lapilli, or by the breaking down and 

 recousolidation of volcanic rocks of various kinds. This consolidation, 

 according to Geikie,* may have taken place either under water or on 

 dry land ; in either case they are as a rule distinctly stratified. Those 

 of the tuff's which are formed from Tertiary or post-Tertiary erupted 

 materials are naturally but slightly consolidated, soft and easy to work. 

 It follows, almost as a matter of course, that they will absorb a propor- 

 tionally large amount of water, and hence be less durable in the exceed- 

 ing trying climate of the Eastern and Northern States. 



The older tuffs are often so firmly compacted that recourse to the 

 microscope must be had to determine their fragmental nature. 



(b) Varieties of Tuffs. 

 According to the nature of the lava, from the disintegration of which 

 the tuffs are formed, they are designated by special names. Bhyolite 

 tuff' is composed of disintegrated rhyolite; trachyte tuff of disinte- 

 grated trachyte, etc. 



(c) Localities and Uses. 



These rocks are very abundant throughout our Western States and 

 Territories, but are scarcely at all used for building purposes, owing in 

 part to the newly settled condition of the country in which they occur 

 and in part to their state of incomplete consolidation. They are, how- 

 ever, soft, and easy though rather unsafe working stones, owing to lack 

 of definite rift and grain, often plucky fracture, and the presence of 

 numerous dry seams and clay holes. They are, moreover, light, frequent- 

 ly weighing only from 75 to 100 pounds per cubic foot, though moderately 

 strong. When not exposed to too wide variations of climate they must 

 prove very durable. Although no systematic experiments have as yet 

 been made, appearances indicate that they would prove extremely re- 

 fractory in case of fire.f 



They present a great variety of colors ; white, gray, pink, red, lav- 

 ender, salmon, green, and even black, are common. 



With these qualities there seems no reason for their not proving a 

 valuable material in dry climates for all kinds of structural purposes 

 where only the rougher kinds of finish are employed, their textures be- 

 ing almost invariably such that they will not polish. 



The light gray and pink rhyolite tuff occurring in Douglass County, 

 Colo., has been used in the construction of the Union Depot, Windsor 

 Hotel, and other buildings in Denver. 



* Text-Book of Geology, p. 1(54. 



t Newberry .state's that the tuffs found near Challis, Idaho, are of "considerable 

 importance as they are extensively used in place of fire-brick for lining lead-smolting 

 furnaces," being very refractory and easily dressed into shape with an old ax. — 

 Trans. N. Y., Acad. Sci., Dec, 1881. 



