Port Clarence Limestone, Alaska. 341 



are supplanted in the section by 300 feet, more or less, of light 

 gray limestones which are mostly magnesian, and are exposed 

 along the crest of the mountain east of Cassiterite Creek. These 

 light-colored limestones are terminated abruptly by a black 

 slate and thin-bedded limestone series. The only fossils seen 

 in these light grey limestones were a few stromatoporoid corals. 



Station No. 7 is located near the head of the south branch of 

 Tin Creek at its junction with a ravine from the east. The 

 brachiopod fauna of station 7 occurs in a hard blue-gray lime- 

 stone stratum a few inches in thickness which lies just above beds 

 with numerous fucoid-like impressions. Brachiopods belong- 

 ing to a single species are present in abundance in this band of 

 limestone, but they are extremely scarce or absent for many 

 hundred feet above and below it. Several hundred feet of lime- 

 stones separate this zone from station No. 8. These are chiefly 

 of the dark bluish-gray, fine textured type. A 60-foot band of 

 nearly black limestone is the only conspicuous interruption of 

 the blue limestone series. An igneous intrusion on Tin Creek 

 somewhat interrupts the continuity of the limestone beds 

 between stations 7 and 8. 



The fossils collected at station 7 have been determined by 

 Mr. E. O. Ulrich to be Finkelnburgia sp. nov. indistinguish- 

 able from a species occurring in the Roubidoux formation of 

 the Cambrian in Missouri. Concerning the age of these two 

 faunas Ulrich states* that, — 



" I feel no hesitancy in saying that Nos. 7 and 8 are older than 

 Beekmantovvn (basal formation of the Ordovician) and younger 

 than middle Cambrian." 



In other words, they are of Upper Cambrian age. While each 

 of these faunules is of Upper Cambrian a#e, it is not equally 

 clear from the faunal evidence, in the opinion of Mr. Walcott, 

 which is the older of the two. The assumption that 7 is the 

 older would involve less complicated structural relations, and 

 appears, from the writer's knowledge of the section, the more 

 probable as well as the simplest interpretation of the succession. 



Two other stations from which the writer collected fossils in 

 the Port Clarence limestone lie near the coast. These lots 

 were determined as Ordovician. They appear to belong in the 

 same general zone which is represented by previous collections 

 which were also determined as Ordovician,' made by Wash- 

 burnef at Merrill Mountain, and Collier:}: at Locality 26, 

 Seward Peninsula. 



The collection made by the writer at Cape York (station No. 9) 

 is represented by a single species which is present in abundance 



* Letter to the writer, Mar. 10, 1909. 



t Bull. U. S. Geol. Survey No. 328, p. 78, 1908. 



lOp. cit., p. 75. 



