ACADEMY OF SCIENCES] BASIC IGNEOUS ROCKS. 7 



No. 1] 



On the one hand they are associated with strongly alkaline phonolites, nephelinites, basanites, 

 and other felspathoid-bearing rocks, while on the other hand they are also accompanied by 

 felspar-basalts often indistinguishable from those accompanying the subalkaline andesites, 

 but they are not commonly associated with gabbros, diorites, etc. These alkaline plateau- 

 rocks occur in regions of vertical block-faulting, with perhaps some tdting but not strong 

 lateral thrust, and consequent folding. They are, however, sometimes found in areas adjacent 

 to regions of central compression, and development of subalkaline rocks, as for example in the 

 case of the occurrence of ouachitites near Mull. It is usually stated that the rocks of this group 

 have been erupted under conditions of crust-tension, but perhaps it would be more in accord 

 with modern views (e. g. of the origin of the Great Rift Valley) to state that the tectonic condi- 

 tions accompanying intrusion were those of relatively slight lateral pressure, or even tension. 



A special subdivision of this group is probably indicated by the perofskite-bearing mica- 

 peridotites, sometimes erroneously termed "kimberlite." These are absolutely distinct from 

 the mica-pcridotite of the Harz as indicated by Rosenbusch ('07) (see p. 20), and from the 

 bulk of "formations ophitoferes" of Sacco ('05), though included by him in that series. They 

 form thin dikes and (rarely) sills in almost horizontal but often faulted sediments in Eastern 

 United States, South Africa, and India. They contain melUite in several instances, and are 

 apparently related to the material of the kimberlite-breccia type of South Africa and the 

 melilite-basalt, and alnoitic breccia of the volcanic necks of southern Bavaria (Schwarz '05). 

 It will be convenient to employ for these the term "alkaline peridotite-dikes. " 



Finally, we have a much more varied group comprising the majority of the spilitic suite of 

 Dewey and Flett ('11). They are stated to be characteristic of those offshore regions which 

 have undergone steady subsidence unaccompanied by folding or faulting. We may therefore 

 consider these as forming on the margin of geosynclines. They comprise sills and flows very 

 closely associated, of picrite, albitic diabase (or dolerite) of several types, spdites, 4 kerato- 

 phyres, etc. Harker has recognized these as a division of the alkaline branch of igneous 

 rocks, with well-marked tectonic and petrographic characters ('11), while Erdmannsdoerfer 

 ('07) and Weber ('10) have shown some petrographical analogy between the essexite, theralite 

 series (our alkaline plateau series) and the diabases and keratophyres of Germany. Rosen- 

 busch's hesitation concerning the affinities of the keratophyres is significant in this connection. 

 Intrusion of the one series into subaerial rock masses, and of the other into sdty subaqueous sedi- 

 ments may have accounted for some structural differences, and Daly ('14, p. 340) urges the later 

 is the cause of the concentration by the action of vapors ("gaseous transfer") of the soda into 

 the rocks of the spilitic suite in which he was formerly supported by Bowen ('10). How far 

 this last may be true or not (and the writer's observations in New South Wales do not support 

 it) 5 the conditions of origin of the spditic suite of rocks, and those of alkaline plateau rocks 

 are alike in the absence of effective lateral compression at the time of eruption, but unlike in 

 the fact that the essexite-theralite group generally were formed in continental regions that 

 have not been folded to any degree subsequent to their eruption, 8 while the spilitic rocks were 

 formed during the slow subsidences in geosynclinal areas that preceded orogenic movements 

 with intense lateral compression, and they are therefore generally greatly disturbed. Daly 

 ('18, p. 112) has objected to a distinction somewhat analogous to this made by Harker ('16) 

 on the ground that it is unsafe to infer the conditions accompanying an act of intrusion by the 

 subsequent history of the region in which it occurred. But, while recognizing that there is no 

 necessary coincidence of tectonic provinces of different ages, some degree of correspondence may 



< These spilites are soda-rich albitic rocks occuring in definite tectonic and petrographic association. There are many non-albitic spilites (e. g. 

 perhaps those of Bohemia described by Slavik '08) which lack these features and can not be included in the "spilitic suite " as defined by Dewey and 

 Flett, though even in these Algonkian pillow-lavas, the origin of the associated chert is assigned to a "post-volcanic effect of the spilitic eruptions!' 

 (Von Purkyne '09). 



11 Compare the statements on page 71, and Sundius observations ('15) in regard to the bearing of Termier's hypothesis ('98) on this point. 



8 Note the exceptional cases mentioned in northwestern Germany. 



