264 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 64 



Rothpletz states that he could not find the localities of Walcott's 

 diagrammatic figures 3 and 4 showing the unconformity between 

 the Cambrian and pre-Cambrian southeast of Helena. I find on 

 looking up my field notes of September 30, 1898, that I crossed from 

 the west side to the east side of the city of Helena, and followed the 

 outcrop of the Flathead quartzite to the southeast and noted that a 

 half mile east of the thickly settled southern section of the city the 

 Helena limestone is not more than 20 feet below the base of the Flat- 

 head quartzite, the interval being filled by arenaceous and argillaceous 

 shales, and that in a prospector's cut about one mile southeast of the 

 suburb of Lenox, the quartzite rests directly on the limestone. At 

 this line of contact there is a slight unconformity between the lime- 

 stone and quartzite that in a distance of 10 feet shows the upper 

 surface of the limestone eroded so as to cut out 18 inches in thickness 

 of the upper layer. A pen and ink sketch was made in the notes and 

 from it the diagrammatic figure 3 was drawn (see fig. 12, p. 286). 

 This particular locality is about 1.25 miles southeast of the suburb of 

 Lenox, near line of section JV-JJ', plate 39. 



The field notes state further that at another prospect cut about a 

 mile to the southeast of the one above mentioned a band of argilla- 

 ceous shale rests on the Helena limestone and the Flathead quartzite 

 rests on the shale. The only evidence of non-conformity here was a 

 small mass of shale left by erosion above the general level of the 

 summit of the shale and rising into the basal bed of the Flathead 

 quartzite. A sketch accompanied this note and from it the diagram- 

 matic figure 4 was drawn (see fig. 13, p. 286). It is quite probable 

 that in the interval between my visit in 1898 and that of Rothpletz in 

 1 91 3, the sides of the two cuts may have broken down by weathering 

 and thus the contact with the basal bed of the Flathead quartzite 

 have been covered over, or it may be that Rothpletz either did not 

 find the particular localities or that in his confused identification of 

 the pre-Cambrian Helena limestone he did not know which lime- 

 stone he was on. These localities are southeast of the fault B-B, 

 plate 39. 



The notes further state that such small unconformities are re- 

 peated on a larger scale along the strike by the increase in the thick- 

 ness of the shales, or by their removal by pre-Cambrian erosion in 

 various places, so that the Flathead quartzite rests in some localities 

 directly on the limestone and in others on the shale. 



This latter statement was based on observations made along the 

 line of contact of the Cambrian Flathead quartzite and the pre- 

 Cambrian rocks, from about 1.25 miles (2 km.) southeast of the 



