200 IRVING. 



portions of the formation to a distance of 1 30 feet below the 

 pinkish hmestone were so generally covered by a heavy talus 

 of limestone, that they could not be seen. In other localities 

 they are exposed, but are so broken and disturbed by intrusions 

 of igneous rocks, that their exact thickness and sequence could 

 not be accurately determined. In the diagrammatic section, 

 Fig. 7, the lower portion was taken from the exposure on the 

 north bank of main Spearfish Creek, just beyond Elmore, and 

 is quite correct ; the upper i 30 feet is from exposures at Crown 

 Hill and Portland, and is of questionable accuracy. A section 

 of the formation, which has been compiled from the various lo- 

 calities is given on p. 199. This section will give an approximate 

 idea of the character of the Cambrian formation in this region. 

 The Silurian shales and limestone are quite persistent in their 

 occurrence, but all of the alternating series below them show 

 great local variations. The exception is the quartzitic conglom- 

 erates at the base, which is everywhere present. No attempt 

 has been made on the map to differentiate the formations lying 

 below the Carboniferous, as they are difficult to separate accu- 

 rately in so disturbed a district. All between the Algonkian 

 and Carboniferous, have been classed as,Cambro-Silurian. 



The Cambrian strata are exposed in the vicinity of Terry 

 Peak and Portland, and in Ruby basin, which lies to the 

 east of Terry Peak. They are too much disturbed by intrusion 

 to afford very reliable evidence as to dip, but the general trend 

 is toward the west. (Plate VIII.) 



The streams of Spearfish, Squaw, Raspberry and Annie 

 creeks, have cut through the overlying Carboniferous strata, 

 and have exposed the Cambro-Silurian beds below. In these 

 exposures the determination of dip is somewhat more reliable. 

 In Raspberry gulch near the forks, the dip is ten degrees west 

 of south, but it shifts gradually around toward the west and 

 finally at the mouth of the gulch, the strata are perfectly hori- 

 zontal. As we descend Spearfish creek, however, we approach 

 the northern slope of the Black Hills uplift and the dip swings 

 gradually around toward the north, and eventually brings the 

 Carboniferous across the stream bed. 



