SUMMARY OF GEOLOGICAL HISTORY 291 



completely removed by erosion or have been entirely absorbed by igneous 

 intrusions. An alternative hypothesis is that these granite pebbles have 

 come from an original granitic earth's crust.^^ 



Summary of geological History 



The earliest events of which we have any geological record in the 

 Province of Manitoba are the extrusion and consolidation of successive 

 floods of lava. In some districts the volcanic periods were preceded by 

 .periods of terrestrial sedimentation; in others, sediments were inter- 

 bedded with the volcanics, and in other areas these early periods of sedi- 

 mentation continued long after the extrusion of lavas had ceased. In 

 places, the volcanic rocks accumulated to great thicknesses; in other 

 places the sediments dominated. 



After the close of the early period or periods of volcanic activity and 

 terrestrial sedimentation and after long periods of induration and meta- 

 morphism, these early rocks Avere intruded by granite batholiths. Deep 

 erosion followed and was succeeded by the deposition of later sediments, 

 such as the Missi series of the Athapapuskow district and the Churchill 

 quartzites, and perhaps certain conglomerates in other areas where the 

 relationships have not been absolutely ascertained. These deposits, how- 

 ever, were all local, probably were not contemporaneous, and deposits 

 found at one locality no doubt correspond to a period of erosion at another 

 locality. 



Then followed a period or periods of mountain -building in which all 

 these rocks were folded and intruded by batholiths of granite. The older 

 rocks were locally sheared into schists, were highly altered near their 

 contacts with the intrusive granite, and were faulted and closely folded, 

 so that today they nearly everywhere stand at a high angle, 



A long erosion era followed the periods of folding and faulting and 

 subaerial processes wore down the mountains to their granite cores. The 

 surface of the country passed from youthful topography through ma- 

 turity to old age, ajid before Ordovician time erosion had proceeded so 

 far that only remnants of the older rocks which once covered the entire 

 region were left, either as synclinal areas, which required a greater 

 amount of down-cutting to reach them, or on divides where erosion was 

 less intense. It was over this peneplaned surface that the Paleozoic sea 

 advanced. 



^» Barren : Origin of the eartti. 



