REPORT OF TEE CHIEF ASTRONOMER 701 



SESSIONAL PAPER No. 25a 



reasons for believing that pyroxene andesite is a direct differentiate of basalt. 

 The greater central-eruptions, like fissure eruptions, seem, therefore, to indicate 

 basalt as the primary eruptible material under part, at least, of each continent 

 and ocean basin. 



3. A tolerably wide study of geological maps and literature shows that basaltic 

 magma is the only one known to recur in all the larger petrographic provinces 

 of the world. That this magma is represented in intrusive diabase, porphyrite, or 

 gabbro, instead of lava flows is, of course, a matter of indifference. Basaltic 

 magma in one of these forms has given rock bodies in each of the alkaline 

 provinces, such as Madagascar, Kola, Bancroft area (Ontario), Montana, 

 Tasmania, Christiania Region, etc. Similarly, no large granitic terrane is free 

 from intrusion of this basic matter. Granite is the only possible rival to basaltic 

 magma with respect to universal occurrence ; yet in the half of the earth's surface 

 covered by the majority of the oceanic islands, granitic or liparitic rocks are 

 unknown. On their thousand-mile zone of fissures the volcanoes of the Hawaiian 

 archipelago, including the greatest in the world, have been built up of essentially 

 basaltic material. Cross's ' biotite-trachyte ' of Hawaii may be regarded as an acid 

 phonolite and. like other alkaline rocks now known in the islands, may be 

 explained as a differentiate of the olivine-basalt type with which it occurs. 



The same law of the steady accompaniment of persilicic (granitic) and 

 alkaline rocks by rocks of basaltic composition has apparently held through all 

 the recognized geological periods since Keewatin-Huronian times. The full 

 significance of this law of distribution cannot be attained until the origin of 

 granitic, alkaline, intermediate, and idtra-basic eruptives is understood. But we 

 may here note that the secular uniformity of the basaltic magma, irrespective of 

 its association with these widely divergent types, can be credited to magmatic 

 differentiation only through an entire disregard of the known laws of magmatic 

 solutions. Since it is also improbable that so constant a type as the basaltic 

 magma is due to assimilation of solid rocks in any other magma, known or 

 unknown, we are left with but the one alternative, that the basaltic magma is 

 primary and of general distribution beneath all the continents and seas. This, 

 too, has been the condition since the relatively late stage in the pre-Cambrian 

 when the Keewatin lavas were poured out on the earth's surface. 



4. Most of the other magmatic types can be explained as secondary and due 

 to the solution of a primary acid earth-shell and of sedimentary rocks in the 

 primary basalt, the syntectics generally undergoing differentiation before the 

 visible rocks were crystallized. The discussion of this thesis forms a large part 

 of the following theoretical sections of the report. It is referred to here merely 

 to point the fact that the non-basaltic magmas of post-Cambrian time at least 

 occur in such volume and relations as are appropriate to the idea here discussed. 



5. Pyroxene andesite, which in volume is probably only second to basalt 

 among the volcanic types, seems to be best regarded as a direct differentiate of 

 basalt. If so, the present argument, so far as derived from an estimate of 

 relative volumes, is strengthened. 



