516 THE DISTRIBUTION, ETC., OP ALKALINE ROCKS, 



sibly shear below. Their extrusion is generally followed by the 

 eruption of basic lavas such as used to be regarded as oceanic. 

 Subsidences in their regions often follow, leading to the formation 

 of senkungsfeld-areas like the Great Rift Valley. 



(4) Lastly, we have the evidence of the oceanic volcanic rocks 

 which often bring to the surface masses of granite, gneiss or 

 gabbro, but seldom fragments of Palaeozoic or metamorphic 

 sedimentary rocks, inclusions of which might be abundantly 

 expected if the oceans had been regions of sedimentation for all 

 time. 



Yet it is quite possible, and indeed very probable, that some 

 of the present land-areas have never been oceanic since the 

 geological record began, and that some of the present ocean 

 depths have never been land; but present depth can only be 

 regarded as poor evidence when we think of the sediments of the 

 deep Tethys and the Alpine Mediterranean now 12,000 to 30,000 

 feet above sea-level in Tibet, the Himalayas and the European 

 Alps. 



The Age op Alkaline Rocks. 



Most of the alkaline rocks whose age it has been found possible 

 to determine were intruded in or about the Eocene period; this 

 point will be more clearly brought out in the more detailed dis- 

 cussion to follow. The fact that foyaitic eruptions attained their 

 maximum at this time cannot be a mere accident. It is also 

 significant that this was a period of gigantic crustal readjust- 

 ments, of mountain-uplifts in some parts, of plateau-uplifts in 

 others; a period of formation of vast subsidence-areas again in 

 other parts and of fragmentation of old continents. Surely these 

 great processes must have had something to do with the produc- 

 tion of alkaline magmas. 



Of course there are many areas of alkaline rock whose age is 

 doubtfully placed in the Palaeozoic, because the invaded rocks 

 are very old. In only one case to my knowledge has a foyaitic 

 rock been proved by means of included pebbles in sedimentary 

 strata to be old Palaeozoic, and that is the Montreal syenite to be 

 referred to later! The alkaline basalts described by Card from 



