HISTORY OF CRYSTALLINE ROCKS. 15 
traverse adjacent schistose strata ; the solid condition of the introducing rock being made 
evident by the accompanying breccia, consisting of its fragments! There is reason to 
believe that such instances are not uncommon, and that in many cases the phenomenon of 
intrusion is due to the superior hardness of the intruding rock, broken beds or masses of 
which are forced through softer strata; the conditions being the reverse of those which 
attend plutonic or volcanic injections. The notion that rocks when in a solid condition may 
be intruded among others, is found in the pages of more than one writer on geological 
questions, but so far as the writer is aware, is for the first time clearly and satisfactorily 
defined in the description of Stapff, which is an important conception gained for the 
student of geognosy. 
§ 21. The endoplutonists, as we have seen, have sought to explain the laminated 
structure of certain crystalline rocks, not, like the exoplutonists, by the pressure attend- 
ant on extrusion, but by movements in an imperfectly fluid material in which, during 
refrigeration, a separation of solid matters and a process of eliquation were going on. 
The possible production in this manner alike of unstratified and stratiform crystalline 
rocks from an igneous mass is ingeniously set forth by Thomas Macfarlane in his studies 
of the geology of Lake Superior.” He notes first, the occurrence of fragments of denser 
and more basic hornblendic aggregates enclosed in lighter and less basic granitoid 
masses, and from these facts, and the composition and specific gravity of granitic veins 
penetrating the masses, conjectures that these various products represent different stages 
in crystallization from a primitive magma, the first-separated portions from which were 
more basic, and the later more siliceous. 
If this took place when the mass was undisturbed, a granitoid rock would be formed ; 
but if while it was in motion, “ hornblendic and micaceous schists and gneisses were most 
probably the results of this process, and the strike of these would indicate the direction 
of the current at the time of their formation.” The material thus separated, notwithstand- 
ing its greater specific gravity, is supposed to have formed at the surface of the molten 
mass, as a result of cooling; but in Macfarlane’s view “there arrived a time when, from 
some cause or other, these first rocks were rent or broken up, and the crevices or inter- 
stices became filled with the still fluid and more siliceous material which existed beneath 
them. This gradually solidified in the cracks, or in the spaces surrounding the fragments, 
and the whole became again a consolidated crust above a fluid mass of still more siliceous 
material,” which by subsequent movements would again be intruded in the form of veins 
in the broken crust. This restatement of the hypothesis of the solidification of a molten 
globe from above downwards, already taught by Naumann,* serves to show how the 
endoplutonist school explains the origin alike of massive and of stratiform crystalline 
rocks, and may be compared with the detailed statement of the exoplutonist view as set 
forth by Poulett Scrope. 
§ 22. The broad distinction sometimes drawn between stratified crystalline rocks, as 
of indigenous and aqueous origin, and unstratified rocks, as intruded or exotic masses of 
igneous origin, thus finds no place in the hypotheses of the plutonic schools, according to 

! See Trans. Roy. Soc., Can., Sec. i. Vol. iv. pp. 112-4, where details and references are given. 
* Geological Features of Lake Superior, Canadian Naturalist, May, 1867. 
* Trans. Roy. Soc, Can., Vol. ii, Sec. iii. p. 10. 
