4 6 



beds become less schistose, and "black slates" intruded 

 by altered dykes of andesitic and gabbroic rocks, predomi- 

 nate. The latter are more prominent in the Juneau 

 district than to the south. At a distance from the Coast 

 Range batholith, intercalated beds of altered lavas and 

 tuffs, usually called greenstones appear; still farther 

 from the contact great thicknesses of these greenstones 

 occur. Such belts of massive greenstone beds are well 

 exposed along Tongass narrows at Ketchikan, along the 

 west side of Cleveland peninsula, on Glass peninsula, and 

 on the west side of Douglas island. 



Beyond this belt and toward the outer coast, the bed 

 rock structure changes with the latitude, and beds of one 

 formation cannot be traced for any great distance north- 

 westerly as can the rocks along the mainland. This is 

 largely due to the irregular island batholithic intrusives 

 noted above. 



The sedimentary rocks flanking the Coast Range 

 batholiths in this region are folded closely near the contact 

 and more openly at a distance, so that, though their 

 general trend is parallel to the range, their dip is extremely 

 variable, ranging from northeasterly to southwesterly at 

 all angles. These dips become more constant, however, 

 toward the north in the Wrangell and Juneau districts, 

 where the schists are more typically developed, where 

 mineralization along certain bands is more pronounced, 

 and where sharp and closed and overturned folds appear 

 to be the rule. The prevailing dip there is steeply north- 

 east into the mountains, and the strike parallel with the 

 range. 



In Southeastern Alaska the oldest rocks are appar- 

 ently a series of fragmental rocks, now represented by 

 banded quartzite, chert, sandstone conglomerate, and 

 some tufaceous material. These clastic rocks grade 

 upward into calcareous beds and limestones containing 

 Silurian fauna. The total thickness of these beds is 

 10,000 feet (3,000 m.) or more. Sedimentation was prob- 

 ably continuous during early Silurian time. Toward the 

 end of the period there was then a gradual deepening of the 

 sea, and several thousand feet of limestone strata were laid 

 down. The oldest member of the Devonian is a succession 

 of conglomerate and sandstone beds composed largely of 

 igneous material, the pebbles of the conglomerate being 

 embedded in a tufaceous matrix and derived chiefly from 

 the older banded quartzite-limestone strata. This series, 



