68 



GEOLOGICAL SURVEY OF THE TERRITORIES. 



Fis:. i8. 



Mexico. I have estimated the aggregate -thickijess of these deposits 

 in this lower basin at the i}resent time at about 1,200 feet. Occupying 

 the area that they do, we can thus see that they possess an importance 

 which demands the notice of the geologist. Between the junction of 

 the Madison with the Jefferson and the mouth of the Gallatin, a dis- 

 tance of about half a mile, there is on the south side a remnant of a 

 iiraestoneridge cut off by the Gallatin from the main mass below, 50 to 

 100 feet high, and trending about northeast and southwest. This is a 

 somewhat peculiar remnant, but it aids much in reconstructing the 

 former surface of the country prior to the great erosion. The Missouri 

 Eiver, immediately below the entrance of the Gallatin, passes through 

 a sort of rift in the uj)turned ridge, the strata on either side inclining 

 in the same direction. How, in the changing of the channel of the 

 river, this remnatit was left, is not clear at this time. There are a num- 

 ber of other remnants of the limestone scattered through the basin, 

 which shows that it was originally scooped out of the series of ridges 

 which probably sent their sharp summits up a thousand feet or more 

 above the general level. Xll the limestones immediately about the 

 junction of the Three Forks are of Carboniferous age, as the fossils 



testify. But above 

 and below, the Silu- 

 rian strata are re- 

 markably well ex- 

 posed. Below the 

 Three Forks the 

 structure, though 

 simple in general 

 terms as above, is 

 very much compli- 

 cated by the chaotic 

 condition in which 

 the strata have been 

 left after upheaval. 

 There are a number 

 of local synclinals as 

 well as anticlinals, 

 but to work them out 

 in as great detail as 

 I desired would re- 

 quire more time than 

 we had at car dis- 



A, Dike of basalt; B. Siipposetl continuation of dike-matter below pOSal. Tnc MisSOUrl 

 the surface; C, Underlying sedimentary beds, probably Pliocene; D, Kiver beiow the iuUC- 

 Pliocene beds somewhat disturbed by the protrusion of basalt. ,-. ' /• . i n^^i 



tion of the Three 

 Forks, flows along a sort of rift in the ridges of limestone, though on 

 the west side these ridges are worn across, so that the waters followed 

 the intervening valley but a short distance. 



On the east side the waters run close up by high limestone-walls for 

 about one-fourth of a mile, and then the valley expands a few hun- 

 dred yards in width for about three miles and then closes again for 

 a short distance, a mile or so, and then again expands to a width 

 of three to five miles. These valleys are evidently worn out of the 

 group of uplifted ridges, as is shown by the remnants here and there in 

 the valley and in the bed of the river. On the east side, immediately 

 below the entrance of the Gallatin, the river has worn a well-defined 



DIKE ON THE MISSOURI. 



