HAMBUEG LDIESTONE. 39 



Secret Canyon Shale.— The Pi'ospect Mountaiu limestonc passes by gradual 

 transition from shaly limestone into brown and yellow argillaceous shales, 

 which, with the exception of one or two thin calcareous layers, present a 

 very uniform character for the entire distance from the extreme southern 

 end of Secret Canyon, where they first crop out, northward until cut off by 

 a fault a short distance northwest of the Eureka Tunnel. Toward tlie 

 upper portion of the series the shale becomes gradually interbedded with 

 thin layers of limestone. The designation of the group is taken from the 

 name of the canyon where it appears most characteristically shown. These 

 beds are recognized only on Prospect Mountain ridge and north of Ruby 

 Hill. The topographical featui-es of Prospect Mountain are largely modi- 

 fied by this shale body, which, eroding more readily than either the over- 

 lying or underlying limestone, has been largely instrumental in determining 

 the drainage channels of the ridge. There are few finer examples of the 

 wealing away of a soft, easily eroded body lying between two harder rock 

 masses than can be seen in Secret Canyon, where the Prospect j\Iountain 

 limestone rises like a wall on one side and the Hamburg limestone nearly 

 as abruptly on the other, while the canyon for over 3 miles is carved out 

 of the shale in a deep, trough-like valley. In their broadest develoi)ment 

 the shale measui-es 1,600 feet, although in places where they are encroached 

 upon by the Hamburg limestone they occur somewhat thimier. As yet no 

 organic foi-ms have been found through the entu-e group, though diligent 

 search was made for them in the more promising calcareous layers. 



Hamburg Limestone.— Transition bcds of shaly limestone, varpng in thick- 

 ness from 25 to 200 feet, pass gradually into the overlying Hamljurg 

 limestone, which forms a prominent, bold ridge between the easily eroded 

 overlying and underlying shales, and, as it is cut through at regular inter- 

 vals by east and west drainage channels, presents one of the most stiiking 

 topographical features of the region, and a geological horizon most easily 

 traced in the field. On the surface this limestone is dark gray, frequently 

 grayish black, and throughout the greater part of the thickness presents a 

 gi-anular textm-e. Layers of fine sandstone and hard chei-ty bands occur 

 at in-egular intervals. In chemical composition it offers no essential differ- 

 ence from the Prospect Mountain limestone, presenting quite as wide a 



