296 The American Geologist. May, 1894 
against it from without. There is no evidence of brecciation 
or of shearing ; and it appears improbable that the parallel 
partings are jointages or faults, since such features are not 
observable elsewhere in the vicinity. We must therefore sup- 
pose that the splitting of the granite into .thin sheets was clue 
directly to the invasion of the diabase magma. It would fol- 
low from this that the parting planes, having once been estab- 
lished by sudden changes of temperature, or otherwise, the 
separation of the granite sheets must have been effected by 
the pressure of the invading magma. The occasional inter- 
section of the granite sheets by the diabase shows that the 
magma invaded the different parallel fissures at the same time, 
and that the process of Assuring and filling by magma was 
not successive but simultaneous. This occurrence has more 
than a local and special interest. The relations of the intru- 
sive rock to the mass invaded are remarkably similar to those 
which obtain between the intrusive Laurentian granite and 
the earlier rocks of the Ontarian system. Geologists familiar 
with the Archaean on the north side of lake Superior will be 
impressed by the resemblance of the photographic illustra- 
tion to many contacts between the Laurentian gneisses and 
granites and the schistose rocks which they invade. Yet 
when the writer first described the evidence of the irruptive 
nature of the Laurentian of the region, the interpretation 
which he placed upon the parallel inter banding of schist and 
granite was ridiculed. It was declared impossible that an 
irruptive contact could assume the characters described, and 
the interbanding was held to be due not to the invasion of 
the schistose rock by the granite magma, but to original sedi- 
mentation in accordance with the views of the old meta- 
morphic extremists. In the case of the irruptive contacts of 
the Archaean the injection of the granite magma in parallel 
.sheets within the schists is not so surprising as is the case 
here recorded. The rocks of the Ontarian system were schist- 
ose prior to their invasion by the Laurentian granites, and 
the parallel planes along which injection took place were al- 
ready established as planes of fission by the cleavage of the 
schists. In the present case there were no pre-existent planes 
of fission in the rock invaded by the irruptive magma. The 
occurrence is, therefore, interesting as establishing independ- 
