216 SCIENCE PROGRESS. 



pitchstone-porphyry and quartz-felsite between two dykes 

 of augite-andesite. In an earlier paper by Judd (18/4) we 

 find a note recording a section at Mingary Castle, Ardna- 

 murchan, where " a mass of quartziferous porphyry has been 

 forced between beds of Lower Lias shale, while a later- 

 formed sheet of basalt has evidently taken advantage of the 

 plane of weakness, constituted by their junction, to force 

 itself between them ". The author compares this with the 

 case of Puy Chopine in Auvergne, described by Scrope and 

 others. 



At certain localities in Skye composite sills of the sand- 

 wich-like type referred to seem to be in connection with 

 composite dykes, which have probably been their feeders. 

 It appears thus that the successive basic and acid intrusions 

 have not only spread along the same bedding-plane, but 

 reached that position by the same channel ; while facts 

 might be cited pointing to an interval of no great duration 

 between the two injections. Such considerations lend some 

 support to the idea that the two magmas, prior to intrusion, 

 existed together in a common subterranean reservoir. If 

 this idea be granted, there is only a step from successive 

 intrusions of different magmas to a single intrusion of a 

 heterogeneous magma. We cannot premise with any cer- 

 tainty the behaviour of an intruded magma drawn from 

 different portions of a reservoir already differentiated ; but 

 phenomena familiar to all geologists clearly point to the 

 existence of what may be termed heterogeneous intrusions. 

 An example involving, not acid and basic rocks, but more 

 or less basic varieties of gabbro, has been studied by 

 Geikie and Teall in the Isle of Skye. 



BIBLIOGRAPHY. 



( This list includes works cited in the former as well as the 

 present paper of this series.) 



(1) IDDINGS, JOSEPH PAXSON. The Eruptive Rocks of 

 Electric Peak and Sepulchre Mountain, Yellowstone 

 National Park. Twelfth Ann. Report U.S. Geol. Surv., 

 pp. 569-664, 1892. 



