550 SCIENCE PROGRESS 



Massachusetts {Bull, 704, U.S. Geol. Surv., 1921, pp. 132), 

 throws much light on such topics as the sequence of volcanic, 

 batholithic, and dyke phases in an igneous episode ; the differ- 

 entiation of subalkalic and alkalic magmas ; origin of shatter- 

 breccias along irruptive contacts ; and the formation of hybrid 

 rocks. Probably the most startling conclusion in the work is 

 that the type essexite of Salem Neck is a hybrid rock due to 

 interaction between an older subalkalic gabbro and a younger 

 nepheline-syenite. It is shown, however, that this hybrid 

 differs in no essential chemical or mineralogical characters from 

 essexites which have certainly resulted from magmatic differ- 

 entiation ; and that hybrid rocks may therefore be identical 

 in some cases with normal igneous types. 



A very detailed study of the igneous geology of the well- 

 known Karroo dolerites of South Africa has been made by Dr. 

 A. L. du Toit {Trans. Geol. Soc. South Africa, 23, 192 1, 1-42). 

 He shows that a kaleidoscopic repetition of sheets and dykes of 

 dolerite occurs intruding the Karroo and older rocks over an 

 area of at least 220,000 sq. miles. The intrusions are dyke-like in 

 the difficultly-penetrable basement of ancient rocks, but are 

 spread out into concordant forms within the horizontal Karroo 

 sediments. From various considerations it follows that the 

 relative order of injection was from the summit down to the 

 base of the sedimentary block. There is a very interesting 

 discussion of the geophysical aspect of the Karroo eruptions. 

 From a study of this and similar areas " it would appear that 

 the prolonged accumulation of sedimentary infillings of this 

 type (Karroo) led to ultimate folding, upheaval, and penetra- 

 tion by basic eruptives on an extensive scale." 



Kinkell Ness, " the largest, best exposed, and most inter- 

 esting volcanic vent along the northern shores of Fife," part of 

 which has been eroded into the famous Rock and Spindle near 

 St. Andrews, has been fully described by D. Balsillie {Geol. Mag., 

 1 91 9, 498-506). Like many of the larger Scottish vents 

 Kinkell Ness has a peripheral zone of non-volcanic debris, 

 suggesting that the funnel shape of these vents is of late origin, 

 appearing when the sedimentary walls began to disintegrate 

 and slump into the central pipe. 



The large double volcanic vent of the Heads of Ayr, finely 

 exposed in both cliff and shore section, has been described by 

 the writer {Trans. Geol. Soc. Glasgow, 16, pt. 3, 1920, 339-63). 

 The infilling material is mainly the subjacent Old Red Sand- 

 stone andesite lavas, although the vent itself is ot Early Carbon- 

 iferous age. It also contained numerous boulders of Iherzolite. 

 The agglomerate is tumultuous and unstratified towards the 

 centre of the vent, but is well stratified, and in vertical beds, 

 at the western margin. The agglomerate is pierced by several 



