Vol. 58.] ANNIVERSARY ADDRESS OF THE PRESIDENT. Ixxv 
the other by injection lit par lit. In the former type the impreg- 
nation is of a most intimate character, so that there is no distinct 
separation of the two elements; in the latter type the compound 
rock consists of alternating folia of granite and sedimentary 
material. Excellent examples of lit-par-lit injection have been 
described by Messrs. Horne & Greenly in their joint paper ‘On 
Foliated Granites & their Relations to the Crystalline Schists in 
Eastern Sutherland.’ Prof. Lehmann’s ‘injected mica-schists’ 
(Injicirte Glimmer-schiefer) illustrate the same point. 
But mixed rocks may also be formed by the injection of one kind 
of igneous rock with another. Basic rocks are often seen to be 
veined and brecciated by acid material, and the gradual passage of 
such complex masses into banded gneisses under the influence of 
differential movement has been described by Prof. Lawson, myself, 
Messrs. Bonney & Hill, and many other observers. Prof. Sollas 
greatly enlarged our conceptions as to this class of phenomena by 
his important paper on the relations of the granite and gabbro of 
Barnavave in the district of Carlingford; and the same ideas have 
still further been expanded by Mr. Harker, Prof. Cole, and 
Mr. Parkinson. We now know that the surrounding rocks may, 
under certain circumstances, be disintegrated and the fragments 
(xenoliths) and individual minerals (xenocrysts) so uniformly 
distributed through the invading magma as to produce mixed rocks 
on a large scale. We are also asked to believe that the process of 
absorption may proceed still further, and give rise to magmas of 
intermediate composition, which in the solidified form may contain 
no visible evidence of the manner in which they have been produced. 
Any account of the evolution of ideas on the subject of contact- 
metamorphism would be incomplete, without a reference to deep- 
seated fumarole or pneumatolytic action. Exhalations of boracic, 
hydrofluoric, and hydrochloric acids have often produced important 
effects, as shown in the development of such minerals as tourmaline, 
topaz, axinite, datolite, and scapolite in the surrounding rocks. 
In the simplest cases of igneous contacts the intrusions have 
taken place after the folding movements have ceased, and the 
contact-minerals sometimes contain records of these earlier move- 
ments in the arrangement of the inclusions; but, in other cases, 
there is evidence of intrusion while these movements were still 
going on. The plutonic rocks, the mixed rocks, and the surrounding 
metamorphic rocks may then receive a common foliation. The 
* Quart. Journ. Geol. Soc. vol. lii (1896) p. 683. 
