EXTENT OF OVERTHRUSTING. 393 



limestone about four miles. If the Shoo Fly limestone is Carboniferous 

 the displacement must be greater. As this determination is based on 

 estimated distances and uniformity of dip, it can only be considered a 

 mere approximation. However, in magnitude it is not extraordinary 

 as compared with the displacement of similar overthrust faults in the 

 northwestern highlands of Scotland* the Rocky mountains of Canada f 

 and the southern Appalachians. % 



The Taylorville fault, as we have traced it across the northern arm. is 

 found to have an irregularly undulating surface, with a very low general 

 inclination southwestward, and is in fact part of the same fault which 

 affects mount Jura. When we compare the total displacement along the 

 Taylorville overthrust (about 4 miles) with the maximum faulting ex- 

 perienced by the overturned strata of mount Jura (three-fourths of a 

 mile) we find the former exceeds the latter over three miles. This dif- 

 ference is large and suggestive. While it is possible that the supposed 

 displacement of the Taylorville fault is too great, yet it is quite improb- 

 able that it is so small as one mile. The Taylorville fault may have had 

 its inception in the folding that took place at the close of the Triassic, so 

 that a lame part of its displacement may be pre-Jurassic. 



The Taylorville fault was formerly regarded as a normal fault,§ but 

 later observations strongly indicate that it is an overthrust. Evidence 

 has not been found to show positively that there has-been any consider- 

 able amount of motion along the Taylorville fault within the later 

 geologic epochs. The Tertiary stream which deposited the Johnson 

 gravels appears to cross the faidt immediately south of the fortieth par- 

 allel, and at that point, according to Mr. Turner,|| the " amount of fault- 

 ing has been comparatively slight." 



Summary. 



There are in the Taylorville region IX sedimentary formations and 17 

 eruptive masses. The former have a total thickness of 24,500 feet ; 17,500 

 feet are probably Paleozoic, and 7,000 feet are Mesozoic. 



Among the sedimentary rocks, one horizon in the Silurian, two in the 

 Carboniferous, three or more in the Trias and five in the Jura have been 

 delinitelv recognized by fossils. 



• "The I Irystalline Bocks of the Scottish Highlands L Geikie, Nature, vol. xxxi, 1881, p. 29 ; also 

 •■ R< porl "ii tli.> l ;.-.- . - 1 1 r Work of the Geologic Suvvey in t lie North wesl Eighlands oi Scotland:" A. 

 Geikie, Quart. Jour. 'Geo! 3oc, vol. xliv, 1888, i>. 378. 



t" Report on tin- Geologic Structure of a Portion of the Rocky Mountains ; " R. G. McConnell, 

 Geol, Survey Canada, Lnnual Report for 1886, pari \>. 



I 'I'll e Overthrusl Faults of ill- Southern Appalachians; C Willard Hayes, Bull, Geol. Soc. Am., 

 vol. J. pp. I ll-l'.l, i.l.-. 2 ni'l 3. 



\ V . s. Geol. Survey Bulletin no 33, p L3; .-ii-<> Eighth Ann. Rept. CJ, S, I I. Survey, p. 126. 



II. w. Turner; Mohawb Lake Beds: Bull. Phil. Soc. of W tshington, vol. xi, p. 106. 



I. Ill I'.i 1 1.. Geoi . Boc. A.m., \ ot, 3, 1891, 



