George M. Dawson — Erratics at High Levels. 209 



rounded patches, less transparent than the rest, and minutely granu- 

 lar in structure, the transparent parts of which also show bright 

 specks with crossed Nicols. There are many dark-brown ferruginous 

 stains (some rather resembling altered ilmenite), a few small quartz 

 grains, and numerous felspar crystals. Most of the last are ortho- 

 clase, rather resembling sanidine, but plagioclase is also present. 

 They have often ragged edges, and a broken or crushed aspect, are 

 sometimes almost ojjaque from ferruginous stains, and contain, 

 minute ferruginous microliths. In one or two of the larger crystals 

 irregular groups of ill-defined microliths are included, resembling a 

 devitrified glass, — possibly these may have once been glass enclosures, 

 — also rather irregular aggregates of viridite (? replacing magnesia 

 mica). The quartz is pretty clear, but here and there it contains a 

 few extremely small microliths, and perhaps cavities. 



The structure of the rock, both macroscopically and microscopic- 

 ally, is remarkable, as it bears considerable resemblance to that of 

 some of the streaky ashes including broken felspar crystals from the 

 BorroM^dale rocks of Cumberland. In the field, however, I had no 

 doubt of its nature, and, after many careful examinations of the 

 slide, I feel convinced that the resemblance is accidental. Originally, 

 it was probably a sanidine trachyte (there is hardly enough free 

 quartz for a rhyolite) with a rather variable base, imperfectly micro- 

 crystalline or crowded with ill-defined, microliths (perhaps not very 

 unlike some of the Drachenfels trachyte). A slight amount of brec- 

 ciation may have been produced by motion just before complete 

 solidification, and subsequent pressure may have further crushed the 

 rock ; after which the original minerals have undergone considerable 

 alteration. 



From its appearance I should imagine that this rock had solidified 

 at no great depth. It is intrusive in the " Grey Unfossiliferous 

 Slates " of Etheridge — upper part of Middle Devonian — near their 

 junction with the succeeding Pickwell Down Sandstones (Upper 

 Devonian), and is affected by cleavage. Its intrusion must accord- 

 ingly be prior to the close of the Carboniferous Period, when the 

 cleavage was produced, and so probably happened some time to- 

 wards the end of the Devonian Period. 



IV. — Erkatics at High Levels in North-Westebn America — 



Barriers to a Great Ioe-Sheet, 



By George M. Dawson, F.G.S., 



Geologist to H.M. Norfh-Americaii Boundary Commission. 



IN the last edition of his Great Ice Age (1877), Mr. Geikie has 

 noticed at some length my descriptions of the glaciation and 

 superficial deposits of the central region of North America, from 

 Lake Superior to the Eocky Mountains ; ^ giving an outline of the 

 facts recorded, and of the explanation of these facts which I have 

 ventured to present as that which appeared to me most probable. 

 In one respect, however, some misapprehension seems to have 



1 Quart. Journ. Geol. See, Nov. 1875 ; also Geology and Resources of the 49tli 

 Parallel. 



DECADE II. — VOL. V. — NO. V. 14 



