ORIGIN OF AMPHIBOLITES OF LAURENTIAN AREA 5 
completely recrystallized rock and differs from any of the normal 
igneous rocks. Under the microscope it is identical with an amphib- 
olite described by Teall which was developed by the alteration 
of a diabase dyke where crossed by a line of shearing. In the case of 
these Canadian dykes, however, the amphibolite is not confined to 
that portion which has been clearly subjected to movement but 
forms the whole mass of the dyke. Seeing that this typical granular 
amphibolite can be proved to have originated from the alteration of a 
basic igneous dyke—in all probability originally a diabase—it is very 
highly probable. that many other occurrences of this rock whose 
origin cannot be determined from their field relations may also be 
derived from the metamorphism of similar igneous intrusions. 
Third mode of origin.—Amphibolites which are identical in 
physical character and in composition with those of class two are also 
produced by the metamorphic action exerted by the granite bathyliths 
on the limestones through which they cut. ‘This is a remarkable fact 
and one which at first sight seems scarcely credible. It is, however, 
a change which has undoubtedly taken place on a large scale. The 
discovery that amphibolite originated in the manner just referred to 
explains what was a very puzzling fact in the early stages of the field- 
work in this region, namely, that while the granite bathyliths break 
through the limestone in all directions, they were filled with amphib- 
olite masses and not with limestone inclusions. ‘This fact was at 
first thought to be due to the granite happening to intrude portions 
of the limestone bands which were impure and thus held within 
themselves the material for the production of amphibolite by diagenetic 
rearrangment; but as occurrence after occurrence over the whole 
vast area was found to present the same phenomenon, it became evident 
that it was impossible to consider that the limestone strata had always 
happened to be impure at the places where the granite had broken 
through them, while elsewhere over great tracts the hmestone contained 
little or no impurity. A critical study was therefore made of certain 
localities where the contact of the two rocks was well exposed and where 
the effects of the intrusion could be studied over a considerable range 
of country. This study showed conclusively that the limestone along 
its contact with the granite became impure through the development 
in it of bisilicates and plagioclase feldspar, and eventually when in 
