20 Annals of the Carnegie Museum. 



Base of gray Madison limestone. 



I and 2. Yellow sandy limestone and shale 79 feet. 



3. Black coaly shale 7 



4. Gray nodular limestone 8 " 



5. Green fissile shale 43 



5a. White thinly bedded limestone partly stained with limonite 20 " 

 5b. Purplish white thinly bedded limestone. 



Remainder of section obscured. 



This completes the list of measured sections of the Three Forks 

 Formation in the northern part of the quadrangle and the neighboring 

 region to the north. A comparison of these sections shows the per- 

 sistence of all of seven members in all parts of the region. The 

 members show in the different sections a considerable variation which 

 may be briefly summarized as follows: 



Members i and 2 together vary from 60-80 feet and have an average 

 thickness of about 70 feet. Member 3 varies in thickness from about 

 19 feet, in the west Three Forks valley, to 6 inches at Logan. It has 

 an average thickness of about 6 feet. Member 4 varies from 3 to 10 

 feet in thickness; member 5 from 50 to 120 feet; member 6 from 15 to 

 40 feet, and member 7 varies from 40 to 80 feet. 



These thicknesses of members i and 6 are much greater than those 

 :given by Dr. Peale in his section for the formation. Dr. Peale's 

 figures have apparently been adopted by Dr. Kindle- in his section at 

 Logan, Mont., and also by Dr. Raymond,^ who, although he did not 

 measure the section here or north of Three Forks, noted the presence 

 of (i) a Lower Red-Shale zone; (2) a Green-Shale zone, and (3) a White 

 Blocky Shale, all part of Dr. Peale's Green Shales. The writer has 

 noted in Section J the probable position and thickness of these zones 

 as recognized by Dr. Raymond. Although these zones are indicated in 

 Sections G and I " farther north " and in Sections M and O south of the 

 Jeflferson River, they are generally not clearly defined and therefore 

 are not given a place among the seven members of the formation as 

 recognized by the writer. 



The slight thickness of the yellow sandstone and shales of members 

 I and 2, noted in Section J, is probably partly due to obscured contacts, 

 and somewhat to actual thinning of the strata. Some deformation 

 in the strata due to the folding and overthrusting may also be the 

 cause of the lessened thicknesses of the members in this section. 



2 Kindle, E. M., Bull. Am. Pal., No. 20, p. 8, 1908. 



'Raymond, P. E. Amer. Jour. Set., Ser. IV, vol. XXIII, pp. 116-122, 1907. 



