554 GEORGE DAVIS LOUDERBACK 



that they are believed to have been deposited under similar conditions 

 in the same sea during the same period. According to the above 

 principles, therefore, they should be given the same name. 



That there is another point of view, however, is shown by the fact 

 that the author of the term "Myrtle formation" has used it in the 

 maps and texts of three different folios, and in other papers, with the 

 distinct understanding and belief for which he presents evidence, that 

 the rocks so designated were formed in the same sea, at the same 

 period, and have the same stratigraphic limits and characteristic 

 fossils as the Shasta. No reason has been given, as far as known, 

 for coining and using the local synonym. 



In the writer's opinion, therefore, the best and most practical 

 interests of science would be served, in the present case, by referring 

 to and mapping the formations under discussion as the Franciscan 

 series and the Shasta group, respectively, of the Roseburg quadrangle, 

 or of whatever quadrangle may be under consideration. If, however, 

 for some reason or other, local terms may seem desirable for the 

 Oregon field, it is believed that the names used in the above discus- 

 sion represent natural groups, and have been sufficiently definitely 

 defined to serve a useful purpose. 



SUMMARY OF RESULTS 

 The rocks of southwestern Oregon which, in the Roseburg, the 

 Coos Bay, and the Port Orford quadrangles have been mapped as 

 the "Myrtle formation," and considered to represent a period of 

 continuous sedimentation corresponding to the Cretaceous from the 

 base of the Knoxville to (at least) the top of the Horsetown, inclusive, 

 are divisible into two natural groups, which differ in their lithological 

 and other characters, and which are separated by an unconformity 

 representing a period of considerable geologic activity. 

 - The older (Dillard) series is pre-Knoxville and of considerable 

 thickness (not measured, but perhaps 8,000 to ic,ooo feet), and gives 

 evidence of a sea in which were characteristically developed radio- 

 larian cherts in series of regularly recurring thin beds, alternating 

 with a peculiar ferruginous shale; thick-bedded, arkose sandstones, 

 almost entirely devoid of fossils; and occasionally a foraminiferal 

 limestone. Less characteristic shales and conglomerates were also 

 formed. These deposits were made in Franciscan time, when the 



