STRUCTURAL FEATURES OF THE P RE-CAMBRIAN 381 



and all of it so inferential that it is of small value. The presence 

 of the great thicknesses of limestone in the Grenville series is, how- 

 ever, strongly suggestive of the existence of lime-precipitating 

 organisms even at that early period, although it is of course con- 

 ceivable that the precipitation of lime at that time may have been 

 chemical. The common presence of scales of graphite in the quartz- 

 ite and crystalline limestone and the also common presence of H2S 

 apparently included in the limestone crystals are, however, both 

 rather strongly corroborative of an organic origin. Whether life 

 extended to the land at this time is more doubtful. If it did, it 

 must have been of a very sparse nature, with little restraining effect 

 on the movement of soils under erosive influences. The nature of 

 the clastic sediments that were deposited throughout Grenville 

 and Mattagami time indicates conditions under which the soil was 

 removed from the land surfaces without the prolonged weathering 

 and thorough decomposition of the mineral constituents that a good 

 covering of vegetation promotes. 



The climate of the Grenville period may be inferred to have been 

 mild for the following reasons : ( i ) If the limestone of the series is of 

 organic origin, it would imply the existence of a warm epicontinental 

 sea. (2) The Grenville sediments, though not well weathered, are 

 as a rule much more acid than the basic lavas from which in all 

 probabiUty they were largely derived. This would indicate, in 

 the absence of vegetation, a warm climate favoring the rapid 

 decomposition of the rock constituents. (3) The enormous 

 extrusions of lava which took place preceding the deposition of the 

 Grenville must have been accompanied by the exhalation of huge 

 volumes of carbon dioxide into the atmosphere. The presence of 

 this carbon dioxide may have had some effect in ameliorating the 

 climate of the period. 



The climate during the Mattagami period is more in doubt, 

 but it was probably cooler than that of the Grenville period. The 

 presence of limestone in very small quantities only might suggest a 

 sea colder than that of Grenville time, in which lime-secreting 

 organisms did not exist in such profusion. The elastics, though 

 frequently similar to the Grenville, are in general more basic and 

 less weathered,. This might also indicate colder climatic conditions. 



