134 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



In the above section the top of the Blue Limestone was possibly not 

 reached, as it forms the surface of the hill, and may have been partially 

 removed by erosion. The thickness given of 130 feet is much less than is 

 found in the vicinity of Leadville. It is readily seen from the varying 

 character of the beds at the base of the Silurian and at the top of the Cam- 

 brian that, in the absence of paleoutological evidence, it is difficult to draw 

 a definite line between the formations. These beds were deposited at a 

 time when the.general character of the sediments was changing from sili- 

 cious to calcareous, and the rapidity with which the change progressed natu- 

 rally varied much within comparatively short distances. The Red-cast bed, 

 of which a specimen from this section is figured in Plate V, is the only one 

 whose character is found to be persistent over the whole area, and this has, 

 therefore, been adopted provisorily as the top of the Cambrian. The aver- 

 age strike of the beds is north and south, and the dip varies from a very 

 low angle to 25 east. 



The rock of each of the porphyrite beds is of the typical hornblende 

 variety figured on Plate VII, Fig. 2. As in other sections, while the 

 porphyrite is continuous on a large scale throughout certain horizons, in 

 detail it is found to be very variable in form, now ending on one bedding 

 plane in a tongue, around which broken masses of the sedimentary beds are 

 distributed like material pushed before the end of a lava flow, and then 

 continued a few feet farther on another bedding plane. Again it appears 

 in small transverse dikes, probably offshoots from the interbedded sheets. 

 Of these the most prominent is at the horizon of the Red-cast beds, standing 

 vertically, with an east and west strike, and 10 feet thick. There are two 

 main sheets of porphyrite. The upper one is only two feet thick in the line 

 of section, and occurs between the Parting Quartzite and White Limestone. 

 As it rises with the slope of the beds to the westward it gradually thickens, 

 becoming 17 to 20 feet thick at the point where it reaches the top of the 

 cliff, and here occurring between the Blue Limestone and Parting Quartzite. 

 The second sheet occurs in the upper part of the Cambrian, being only six 

 feet on the line of section, but thickening to the eastward. 



The rock of the porphyry sheets is so much decomposed that it cannot 

 be definitely decided whether it is more closely allied to the Lincoln or to 



