70 GEOLOGICAL SURVEY OF THE TEREITOEIES. 



Siliuiau beds extend I do not know, but they were visible at least fifty 

 miles below the junction of the Three Forks. It will at some future 

 day be au interesting and profitable task to study the geology of this 

 region in detail. The few facts that I am able to present at this time 

 may serve to call attention to it. There is here the largest development 

 of the Silurian strata I have ever seen in the West. They are, however, 

 only shown over restricted areas. There seems to have been very little 

 system in the upheaval of the rocks. Here and there they are brought 

 to the surface. The lake-basin is about five miles long, and on an average 

 two miles wide, and the aggregate thickness of the modern deposits 

 about 800 feet. 



On the west side of the Missouri, immediately below the Three Forks, 

 there is a most interesting synclinal, which will be shown in the illustra- 

 tion. The river, as it enters the caiion at the Three Forks, may have 

 started in a rift, but it seems to have immediately disregarded it and 

 cut across the ridges, so that the channel now cuts diagonally across 

 all the ridges of the Carboniferous series and a large portion of the 

 Jurassic within a distance of ten miles. The ruggeduess of this region 

 is very great, the little branches that flow from the hills on either side 

 gashing deep canons, exposing the strata in high walls on either side. 

 By following one of these little streams from the main river up to the 

 divide, we get a fair section of the strata. About five miles below the 

 Three Forks, on the west side, the Jurassic ridges come in close to the 

 river-side. In the ridges of limestone above this point is an abund- 

 ance of characteristic Carboniferous fossils, so that we regard the age 

 of these rocks settled. Above them comes a remarkable series of 

 quartzites and sandstones, with intercalations of sandy clay. The lime- 

 stones seem to pass gradually up into quartzites, so that the upper beds 

 are compact, brittle, gray quartzites. The want of continuity of the 

 strata between the Jurassic series and the Carboniferous group below is 

 shown in the absence, in most cases of some hundred feet, of strata 

 which are well shOwu here. The first ridge of quarzite is about 300 

 feet thick, with a dip of 45'^. In the lower part is an intrusive bed of 

 igneous rock. The second ridge is a rusty-brown sandstone, with layers 

 composed of fossils, mostly fragments, as Ostrea, and some beautiful but- 

 undescribed forms. The inclination of the strata is 35° to 40°. There 

 is in this ridge a remarkable intrusive bed of igneous rock, very irregu- 

 lar in thickness and horizontal extent, sometimes 50 to 100 feet thick, 

 pinching out and then re-appearing in full force. The calcareous sand- 

 stones above and below are full of fossils, and do not «eem to have been 

 affected by contact. There is a kind of cleavage in the. igneous rocks 

 that gives to the entire mass the appearance of stratified rocks, but 

 precisely opposite in inclination to the sedimentary beds which inclose 

 them. Then comes a series of beds weathering a dhll, purplish color, 

 composed of sandstones, quartzites, with loose clays and shales, passing 

 up into brown sandstones, then a bed of dark-brown quarzites. Then 

 comes a series of layers of reddish-yellow sandstones of various tex- 

 tures, with intercalations of arenaceous clay. Still farther above are a 

 series of red and purplish clays with greenish-blue bands passing up 

 into gray marls and arenaceous limestone. The aggregate thickness of 

 the mass of Jurassic strata was estimated to be about 1,500 feet. The 

 direction of the dip is about northwest. Then comes a series of brown 

 and rusty-yellow arenaceous clays and sandstones 500 to 800 feet in 

 thickness, with an abundance of well-defined Cretaceous fossils. In 

 the middle of the synclinal is a limited thickness of the Coal strata, 

 with layers of impure coal. At the junction of the two sides of the 



