BLACK HILLS GEOLOGY. 229 



2. Sheets in the Cambrian. 



Second only in importance, and in number far superior to the 

 laccolitic masses, are the sheets which occur in the Cambrian 

 formation. So great is their number throughout the district 

 that it is no exaggeration to say that the thickness of the Cam- 

 brian formation has been increased by nearly one-fourth in 

 those localities which are near the centers of eruption. Crosby 

 has remarked^ with great truth : " It is no uncommon thing to 

 find, as in the district on the east side of Terry peak, known as 

 Ruby Basin, from four to six intrusive sheets in one continuous 

 exposure ; the thickness of the Potsdam partings, in some cases, 

 scarcely exceeding that of the eruptive layers." The sheets 

 vary from less than a foot through all thicknesses up to 200 

 or 300 feet, until we pass by imperceptible gradations into those 

 masses that can be more properly described as laccolites. They 

 vary greatly also in their regularity and persistence, sometimes 

 being short, thick and irregular, and again long, thin and of 

 great lateral extent. The more extended and persistent sheets 

 lie between the heavy and less easily broken members of the 

 Cambrian series, while those which assume a very irregular 

 form are most commonly in the thicker horizons of very fine 

 shales, where fracture is equally easy in all directions. 



The largest and most persistent sheet of the district is that 

 which is exposed on the sides of Squaw creek. It originates 

 in a very large dike-like mass of trachytoid phonolite, nearly 

 opposite the mouth of Redpath creek, through which the 

 stream has cut a deep, narrow pathway with precipitous sides. 

 From here it may be traced up along the sides of the creek 

 almost to the mouth of the west branch of Labrador gulch, 

 where, following the dip of the Cambrian, it has mounted high 

 up on the side of the divide. On the opposite side of Squaw 

 creek it cannot be traced so far south, but has attained a much 

 higher elevation by reason of the westerly dip of the shales 

 and sandstones. 



Measurements of the thickness of this sheet were not made, 

 but it is probably not less than 200 feet in the thickest 



' Proceedings of the Boston Society of Natural History, Vol. XXII, p 512. 



