576 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



O 8. ATLIN LAKE, BRITISH COLTJMBIA. 



Gwillim/** in reporting on the Atlin Lake gold district, describes an extensive 

 area of sandstone and conglomerate about the southern end of Taku and Atlin lakes. 

 The rocks are classed as Mesozoic but were not seen in contact with Paleozoic forma- 

 tions. " Their origin is chiefly igneous and they often pass imperceptiby from a 

 sedimentary and stratified form into the mountain masses of porphyrite and andesite. 

 The few fossils found in some of the bedded sandstones appear to belong to the 

 Jurassic period." This general statement is followed by a more detailed description, 

 as follows : 



The prevailing variety of this sandstone series is of a greenish-gray color. It is usually 

 in heavy beds. There are occasional bands of a darker and more argillaceous-lookiag material, 

 also some thick deposits of thinly bedded fime-grained black and gray material. Conglomerates 

 occur somewhat rarely. Such beds usually contain very coarse bowlders, as large as 3 feet in 

 diameter. These bowlders are principally granite, with sometimes a considerable number 

 consisting of crystalline limestone and porphyrite. 



A very good section of this sandstone formation is found on the west side of Tory Inlet on 

 Atlin Lake. An anticliae occurs at this point. The northern slope of the beds forms the 

 abrupt eastern face of Section Mountaia. This section shows over 5,000 feet of thickly bedded 

 sandstone of the greenish-gray false-bedded variety interbanded with some finer-grained beds 

 of darker material. The upper portion of these beds is without conglomerate. The lower beds 

 contain some narrow bands of which the bowlders are usually small and consist principally 

 of granite and porphyrites (hornblende and andesite porphyrite). 



From microscopic examination of several specimens of this series, they appear to be of 

 pyroclastic origin. 



A few fossil forms were found in some of the darker fine-grained beds of this series of rocks. 

 * * * From field conditions and lithological resemblance, this series of rocks was at first 

 believed to be Cretaceous in age. The examination of the few fossil forms appears to place 

 them in the Jurassic. * * * 'pjjg following is a note on the specimens of fossils collected on 

 Atlin Lake during the season of 1900 and submitted to Dr. H. M. Ami, for examination: 



"The fossils are preserved for the most part ia a rather imperfect manner in a dark, at times 

 streaky, gray fine-grained calcareous rock, which, when examined in thin sections under the 

 microscope, reveals the structure of a porphyrite or andesite tuff. * * * The fossils are for 

 the most part fragmentary * * * b^t they represent several small collections * * * 

 and possibly different horizons in the Mesozoic. It is very difficult to state precisely what is the 

 age of the strata, * * * both on account of the condition ra which the fossils are themselves 

 preserved and on account of the fact that the fauna represented is practically a new and hitherto 

 unrecognized one in that portion of North America. * * * The presence of a few ammonites, 

 which had the general outward appearance of Arniotites not unlike A. vancouverensis seemed to 

 indicate a similar horizon to that of the Triassic system of the Cordilleran belt, but as none of 

 these ammonites show any of the sutures, it is impossible to state precisely in what section or 

 division to place them." 



Some of the most typical of these fossils were sent to Dr. T. W. Stanton, of Washington. 

 He found it difficult to determine the ammonites even generically, since they showed no sutures. 

 He says: 



"These may possibly be Triassic, but I think it more probable that they are early Jurassic. 

 They are certainly not as late as the Cretaceous." 



P 8. LEWES RIVER, YUKON PROVINCE. 



For a description of the conglomerate and sandstbnes near Lake Labarge, 

 which Stanton considers probably Jurassic, see Chapter XV (p. 701). 



