386 GEOLOGY OF "THE YELLOWSTONE NATIONAL PAEK. 



NATURAL BRIDGE. 



Bridge Creek has received its name from a small natural bridge of 

 rhyolite which spans a narrow gulch through which runs a tributary to the 

 main creek. This bridge, which is shown in the illustration (PI. XL VIII), 

 consists of two vertical slabs of lithoidal rhyolite, parts of the contorted 

 layers of lava flow, which stand about vertical in this place. The vertical 

 layers just east of the north end of the bridge are shown in PI. XLIX. 

 They are slightly curved and are separated by open crevices with rough- 

 ened scoriaceous walls. Of the two slabs forming- the bridge the eastern, 

 or that seen in the illustration, is 2 feet thick at its ends and thinner in the 

 middle. There is a space of 2 feet between it and the western slab, which 

 is 4 feet thick. The span of the arch is about 30 feet and its rise about 10 

 feet, the top of the bridge being some 40 feet above the creek. The eastern 

 slab is traversed by two vertical cracks, and by horizontal ones just below 

 the base of the arch. The rhyolite is porphyritic and lithoidal, dark blue- 

 gray, mottled with light gray, and distinctly banded in places. It bears 

 numerous hollow spherulites of considerable size and many small lith- 

 ophysfe with delicate concentric shells, but no small megascopic dense 

 spherulites. The lithoidal rock alternates with glassy layers of black 

 perlite having dense spherulitic bands and some large dense spherulites 

 (2048 to 2052). 



The large hollow spherulites have been crushed while the matrix was 

 plastic, though not liquid, for the broken shells have been dislocated and 

 the sides of the spherulite ■ forced in and the cavity partly filled by the 

 matrix. But this was not liquid enough to enter very far into the hollow 

 cavity, nor has it filled up the cracks on the outside of the shells. It is 

 evident that there was motion in the lava after the large hollow spherulites 

 had formed, and that they were rigid crystalline bodies. It is quite as 

 evident that the delicate lithophysse of various sizes were not formed before 

 the lava came to rest, because they have not been crushed in any case, 

 although their shells are often much thinner than those of the hollow 

 spherulites. Moreover, their eccentric and irregular shapes are more or 

 less in accord with the crooked and distorted banding which marks the 

 planes of flow in the rock. They have the same character as those at 

 Obsidian Cliff and are highly ciystalline ; but the fayalites have been 

 changed to light-yellow opaque pseudomorphs, and the iron has been 



