EFFUSIVE IGNEOUS MASSES 311 



In general terms : Strata underlying igneous masses, parallel 

 with them, are always older than those masses, but strata lying 

 directly upon such igneous rocks must be considered older only if 

 they have been altered or in any way affected by the igneous mass 

 while the latter was still hot, but younger if they are not affected 

 and especially if they contain worn fragments of the igneous rock. 

 An exception must, however, be made in the case of strata de- 

 posited upon a hot laval stream, the heat of which later aft'ects the 

 strata of younger age. In such a case evidence Avithin the strata 

 or the igneous mass will generally furnish the clue to the respective 

 ages of each. Misinterpretation of the contact between adjoining 

 formations may result in grave errors regarding the relative ages 

 of these formations. Thus up to within very recent times the great 

 granitic and gneissic masses of the Canadian shield known as the 

 Laurcntian were believed to represent the oldest known rocks of 

 the earth's crust, the metamorphosed rocks now called the Keewatin 

 series being regarded as the next younger, and representing ancient 

 sediments resting unconformably upon the Laurentian. It is now 

 known, however, that in many instances at least the contact between 

 the Laurentian and the Keewatin is an igneous contact, the former 

 being intruded into the latter, and hence younger than it. 



Effusive Igneous Masses 



Effusive masses, i. e., volcanic flows or sheets are always 

 younger than the strata on which they rest and older than the 

 strata which overlie them. Volcanic sheets are distinguished from 

 sills, which are younger than both the enclosing strata, by the fact 

 that only the stratum underlying the flow is metamorphosed, while 

 the overlying one generally contains fragments of the flow. Fur- 

 thermore, the upper portion of the flow shows generally a more or 

 less vesicular structure which indicates the exposed surface of the 

 old lava sheet. 



Features of the Basal Contact of Lava Sheets. The basal con- 

 tact of lava sheets may furnish important evidence of the physical 

 condition of a region prior to its invasion by the lava flow. Thus a 

 land area, bare or covered by soil either dry or with a comparatively 

 small content of water, will give a relatively smooth and uniform 

 contact line with the overflowing lava sheet. A saturated soil, or 

 one covered by water, will give a very different kind of contact. 

 The conversion of the water into steam will cause a violent ebul- 

 lition along the line of contact, accompanied by an intcr-kneading 



