1877.] Mr Marr, On the Phacopidae of the Lake District. 69 



as one or both occurred, wherever these beds had been found, and 

 had not yet been discovered in the Coniston Limestone proper; 

 the two Chasmops again having never been found in the Upper 

 Bala beds. He observed that the conclusion pointed to by the 

 Phacopidae was fully borne out by the other fossils of these 

 two formations, as also by lithological characters, viz., that the 

 Upper Bala Shales were altogether newer than the Coniston 

 Limestone, and were not merely due to local conditions, and pass- 

 ing elsewhere into limestones ; and hence he concluded, that as the 

 shales varied greatly in thickness, in passing from place to place, 

 and were sometimes altogether absent, that they were either very 

 irregularly deposited, or else subsequently denuded, in either 

 case pointing to a break before the deposition of the next over- 

 lying beds. 



The author also remarked on a-Phacops in the graptolithic 

 mud stones, as being of the subgenus Phacops proper, and probably 

 referable to Ph. Musheni, an exceedingly characteristic Silurian 

 form. 



In treating of the Coldwell beds, he observed that these beds 

 seem to have been sometimes confused with the Upper Bala series, 

 being very like them in lithological character, and containing 

 a species of Phacops, of the subgenus Odontocheile, exceedingly 

 like the one [P. (O.) mucronatus var.] in the lower series, but 

 differing in well-marked characters. The Coldwell beds occur 

 across the Troutbeck Valley, on the east side of Windermere, as 

 well as at the typical place on the west side of the Lake, and are 

 high up in the Coniston Flag series. He concluded that the P. 

 mucronatus var. of the lower series had migrated on change of 

 conditions after the end of the Upper Bala period, and had 

 returned to the original area on the recurrence of conditions 

 similar to those of its former habitat, having changed by variation 

 to P. obtusicaudatus in the meantime. 



In conclusion, he referred to the mention of Orthis, apparently 

 O. vespertilio, having been found by Prof. Nicholson in these Cold- 

 well beds (Nich. Essay on Geol. of Cumberland and Westmorland, 

 p. 61), which were undoubtedly Silurian, considerably diminishing 

 the value of this fossil as a test of the Cambrian nature of beds 

 containing it in this district, and that hence much stress cannot be 

 laid on its occurrence in the graptolithic mudstones, as stated by 

 Profs. Harkness and Nicholson in a recent paper at the Geological 

 Society {Q. J. G. S. Vol. xxxiii. p. 473). 



