PETROGKAPHY OF CHENGWATANA SERIES 33 



o 



crystalline secondary minerals, chiefly epidote and quartz. It occurs in 

 the division zones between flows. A volcanic ash was discovered at 

 division planes between flows, a discovery which substantiated in a very 

 convincing manner the subdivision of the series as indicated.* 



A breccia between lava flows occurs whenever the surface of a flow is 

 sufficiently vesicular and ragged to be crushed into a broken mass by 

 the weight and movement of the next succeeding lava stream. The 

 material is therefore excellent evidence of the existence of a succession 

 of thinner flows instead of a single enormous outpouring of material. 

 Locally there would be accumulated an abundant mass of cindery debris. 

 If this be rolled beneath the advancing stream, a gravelly habit could 

 be induced. Such seems to have been the case at Taylors Falls, and the 

 material is in no sense to be regarded as an intraformational conglom- 

 erate, if Doctor Berkey be correctly interpreted. 



Geikie, in his discussion of agglomerates, defines these products as 

 "the coarser fragmentary materials thrown from volcanic vents," f and 

 these again are angular or rounded ; the angular, collected in sheets or 

 strata outside the vent, if coarse is breccia, and if finer is comprehended 

 under the general name of tuffs, while the rounded materials are con- 

 glomerates or sands, according to the state of comminution. Among the 

 earlier explosions of a volcano, fragments of non-volcanic rocks may be 

 included in the pyroclastic detritus. 



At Taylors Falls the brecciated habit of the rocks under consideration 

 is obscure, owing to the hard and compact nature of the rock. Indeed, 

 this rock is fully as resistant as other portions of the mass. Inspection 

 shows that it consists of fragments of diabase, " angular, irregular, and 

 of all sizes, imbedded in a matrix of finely crystalline secondary min- 

 erals, chiefly epidote and quartz.";]; Its place of occurrence is along the 

 division zone between two flows. At other division zones a minerally 

 saturated and now compact ash was subsequently discovered and 

 accepted as confirmatory proof of the accuracy of the earlier conclusion 

 touching the incidental nature of the breccia, and the association of the 

 two is taken as proof of the existence of a series of lava flows at this 

 locality instead of an enormous dike, as is too commonly supposed. 



In quantity the amount of breccia and tuff in the Minnesota localities 

 is small compared with the enormous masses noted by Geikie in his 

 examples § of volcanic activity during the Tertiary volcanic period in 

 Britain, and by Iddings around Electric peak.|| Another character of 



*Ibid., p. 145. 



t Ancient Volcanoes of Great Britain, vol. i, 1899, p. 31. 

 t Berkey, loe cit., p. 145. 



g Sir A, Geikie : Ancient Volcanoes of Great Britain, vol. i, 1897, p. 32. 



II The eruptive rocks of Electric peak and Sepulchre mountain, Yellowstone National Park. 

 12th Ann. Rept. Director U, S. Geol. Survey, 1892, p. 634. 



