234 IRVING. 



northeast, and in the bed of Long Valley creek, may be con- 

 nected with the dike, from which the Elk mountain mass orig- 

 inated, and it is not improbable that this dike is within the mass 

 of Elk mountain. In that sense, then, the latter is laccolitic. 

 It presents, however, no essential difference from the other 

 small, thick-set intrusions in the Carboniferous. 



Other Intrusions in the Carboniferous. 



On the divide between Calamity gulch and a tributary of 

 Spearfish creek, which runs in a northwesterly direction, is an 

 exposure of phonolite of a very irregular character. It outcrops 

 in an abrupt cliff on the edge of the gulch, and from the flats to 

 the southwest is a quite conspicuous point. On the middle of 

 the divide it is covered by limestone, but is exposed again 

 around the head of the gulch. To the west, on the edge of the 

 cafion, and directly across from Spearfish Falls, is another mass 

 of the same rock. A third and very irregular mass of tinguaite 

 is seen on the edge of the canon on the Pete Hand flat. Other 

 than these, irregular dikes and porphyry masses occasionally 

 occur in the limestone. Such are especially noticeable in the 

 Ulster mine, which is on the ridge just southeast of Twin Peaks. 

 Here dikes and irregular masses of quartz-porphyry and phon- 

 olite appear, and some of them are of considerable extent. 

 All, however, seem to be connected with the Twin Peaks up- 

 lift. The only other intrusion in the Carboniferous worthy of 

 note is the large sheet of mica-diorite- porphyry which cuts 

 across from the Cambrian up into the Carboniferous, so as to 

 include a portion of that formation below itself 



B. COMPARISON BETWEEN INTRUSIONS IN THE 

 THREE FORMATIONS. 



Perhaps no more striking feature of the eruptive action in the 

 Northern hills can be found than the contrast in form and dis- 

 tribution presented by the intruded masses as they pass from 

 one formatian to another. 



The litholoeical character of the formation has in each case 



