BLACK HILLS GEOLOGY. 231 



half a mile above Maurice, are two sheets of phonolite. The 

 one nearer Maurice is of a grayish character and indetermin- 

 able extent. The other is some distance farther up stream, 

 and is in the form of a very heavy sheet, intruded just below 

 the Carboniferous limestone. It is extremely thick in places 

 and is best exposed where a small stream has cut through it a 

 very deep, narrow gorge, not more than a few feet from the 

 railroad. It is one of the most typical phonolites of the region. 

 No further igneous rocks can be observed in Spearfish canon 

 until the mouth is reached of Annie creek. Here there is a very 

 thick sheet of a fine-grained phonolite, and the same rock is 

 found in the bed of the creek near the railroad bridge, some 

 distance up stream. Along the railroad, close to Elmore and 

 at other points on the east bank of the creek, are other expo- 

 sures. Ascending Annie creek one encounters two sheets of 

 phonolite, the lower of which may be seen lying with perfect 

 horizontality on the Cambrian shales 'at the mouth of Lost 

 Camp creek. It may be traced up the stream until it disappears 

 beneath the shales a little below Davier's cabin. The upper 

 sheet runs far up into Rose Spring creek on the north and 

 around into Lost Camp on the south. Other sheets and dikes 

 of phonolite occur in this locality also. 



At the head of the stream is the rounded summit of Foley 

 peak, which is composed of quartz-porphyry. Whether this 

 mountain consists of a series of sheets or is one large mass of 

 igneous rock was not determined. 



Lying almost exactly between Foley peak and Green moun- 

 tain is an irregular sheet of phonolite of considerable extent, 

 which extends down to the railroad near Portland, and was 

 evidently a portion of the same mass that forms the crest of 

 Green mountain, but the connecting portions have long since 

 been removed by erosion. 



Beneath the Green mountain mass lie the Cambrian shales in 

 which mine tunnels have been run in all directions toward the 

 center of the hill. None of them, however, encountered any 

 phonolite. 



Sheets of quartz-porphyry are also to be seen on the slopes 



