CHICHAGOF COVE 79 



second ridge, giving a synclinal structure to this part of 

 the area. 



Concerning the fossils collected from this formation, Dr. 

 Dall, to whom they were submitted for study, says : " The 

 fossils from the Stepovak beds are Eocene, probably of 

 about Claiborne age (middle Eocene), and the only typical 

 Eocene yet discovered in Alaska." The description of 

 these fossils by Dr. Dall and a discussion of the place of 

 the Stepovak Series in the Alaska geological column will 

 be found elsewhere in this volume. 



THE IGNEOUS ROCKS 



The Laccolith 



The intrusive rocks which have invaded the beds of the 

 Stepovak Series are in the form of numerous dikes and a 

 large laccolithic mass from which the dikes appear to 

 radiate. The laccolith is well exposed near the summit 

 of Chichagof Peak on the eastern side, the only point of 

 the crest of the plateau which was reached. The dia- 

 grammatic section, fig. 18, shows the relations that could 

 there be observed. A general view to the north from the 

 summit of the mountain gave the impression that the 

 igneous rocks formed an extensive mass, the observed 

 section lying on its extreme southeastern periphery, and 

 therefore giving an inadequate idea of the bulk of the in- 

 trusive matter as compared with the sedimentary rocks. 



The material of the laccolith is chiefly diorite-porphyrite, 

 with small amounts of augite-diorite-porphyrite. It is a 

 dark-colored rock, grey to greenish grey, and rather fine- 

 grained, with distinctly prophyritic texture. On weathered 

 surfaces the phenocrysts of hornblende stand out in relief 

 and give the rock a very rough surface. The character- 

 istics of the rock in its various phases may best be learned 

 from the descriptions which follow of particular specimens. 



