BUILDING STONES. 399 



THE CASTLE ROCK BUILDING STONE. 



This is arhyolitic tuff. It occurs as a bed of variable thickness — from 

 a thin sheet to 200 feet — separating the two divisions of the Monument 

 Creek formation. The deposit nowhere enters the limits of the Denver 

 Basin as represented on the map, but lies wholly to the south, outcropping 



in isolated patches at various localities on the Arkansas divide. The stone 

 is extensively quarried in the neighborhood of Castle Rock, 33 miles south 

 of Denver, and has long been used in the construction of many of the 

 more important buildings of Colorado. 



The rock is a deposit of eruptive material of the composition of 

 rhvolite. Its main constituents are in an extremely fine state of division 

 and form a homogeneous rock mass, except for minute fragments of the 

 crystals of glassy sanidine and small scales of biotite-mica which are 

 scattered through it with comparative uniformity. The rock is rendered 

 more or less porous by numerous cavities, of sizes up to that of a walnut, 

 the result of a decomposition of some accessory mineral component which 

 is distributed through the bed. This feature makes the stone undesirable 

 for employment where great strength is required; for the superstructure of 

 buildings ,,f moderate height its compressive strength is, however, ample. 

 In color there are two distinct varieties, a delicate shade of pink and 

 a bluish-gray of equal depth. Either may he used alone, or they may 

 be combined without order in the general structure, or one may be em- 

 ployed in ornamental relief to the other; they may also be effectivelv 

 used in combination with the red Triassic stone from Manitou. The 

 Castle Rock stone retains its color well, and from an artistic view 

 forms one of the handsomest building stones of the West. The stone is 

 usually dressed in the rough or rock finish, its fracture being distinctly 

 conchoidal. The durability of the stone can only be judged from its con- 

 dition in the outcrop. This is such as to warrant belief in a strong resist- 

 ance to weathering, at least in the dry atmosphere of the West. The 

 older buildings of this stone in Denver have had as vet but about 

 eighteen years 1 existence. 



