262 A. H. BROOKS RECONNAISSANCES IN SOUTHEASTERN ALASKA 



by lignite-bearing sediments of Oligocene age, which are only slightly 

 disturbed or indurated. Some volcanic rocks have been extruded in 

 post-Tertiary times. Besides these, dikes of various rock types are pres- 

 ent in all the pre-Tertiary beds. 



The earliest epoch of intense disturbance was in pre-Devonian times, 

 and was accompanied by large intrusions of basic igneous rocks. The 

 next period of metamorphism was in middle Mesozoic times, when the 

 intrusion of the granite of the Coast range took place. The alteration 

 of the sediments adjacent to the mass of granular igneous rock is as- 

 signed jointly to contact metamorphism and the mechanical effect of 

 injections. The granite itself shows the effect of deformation by which 

 it has been locally changed to gneiss and mica-schist. No evidence of 

 any post-Tertiary disturbances has been found. 



The history of the deformation of the rocks of the region is a complex 

 one, and has not yet been deciphered. In the western belt of Lower Paleo- 

 zoic beds the strata are intensly metamorphosed and deformed. To the 

 east in the Upper Paleozoic beds dynamic action has been less, while 

 still farther to the east in the belt of argillites, the metamorphic action 

 has again been intense. To the east of the Coast range the Paleozoic 

 rocks have all suffered about the same degree of alteration, with the ex- 

 ception of those which lie immediately adjacent to the intrusive granites. 

 It is evident that the observed metamorphism has been of two kinds, 

 assignable to different causes — the regional metamorphism, which is due 

 to deformation; and the contact metamorphism, which has been brought 

 about by the intrusion of igneous rocks. 



The phenomena of contact metamorphism are commonly regarded as 

 confined to chemical effects produced by the heat and accompanying 

 gases of igneous intrusions, but many cases are on record where the me- 

 chanical effect caused by the pressure of the invading rock has been of 

 great importance. Such is the case in the vicinity of the granite masses 

 of the Coast range, where the mechanical alteration of the rocks is quite 

 comparable to the regional metamorphism noted in neighboring localities; 

 and, since the intrusion occurred after the greatest regional disturbance, 

 the effects of the latter have been to a certain extent obscured by the 

 former. 



There are three zones in the province which are marked by more or 

 less intense metamorphism. The one includes the rocks of the Lower 

 Paleozoic beds, extending through the western group of the Alexander 

 archipelago, while the other two lie on either side and adjacent to the 

 granite of the Coast range. The two westernmost of these zones, in the 

 southern part of the region at least, are separated by a belt of Upper 

 Paleozoic beds, which are folded, but only slightly indurated, while the 

 Coast Range granite separates the two eastern zones. 



