PETEOGEAPHY OF 0:nTAEIO. 57 



this mineral forms slender parallel prisms, the rock has two A'ery perfect cleavages and 

 splits into long, roughly prismatic slabs. 



Microscopic. — In the composition of these rocks, hornblende almost invariably plays 

 the chief part, regularly accompanied, however, by a considerable amount of quartz and 

 magnetite in small grains. In most cases also more or less plagioclase is found. Accessory 

 minerals are tolerably common, such as biotite, ilmenite and pyrite or some other sulphide. 

 As products of decomposition, chloritic minerals, oxides of iron, calcite and epidote are of 

 frequent occurrence. These rocks may be classified as : — 



Gneissoid-diorite-schists, when there is comparatively little hornblende. 



Quartz- diorile-schists, which are by all means the most common. 



Quartz-dhrite-porphijroides, or perhaps better, plagioclase porphyroides. The third 

 group is quite common and embraces rocks with a microgranitic or microfelsitic magma, 

 formed, so far as the constituents can be determined, of fine fibres or grains of horn- 

 blende, with particles of quartz and magnetite. This magma encloses crystals of plagio- 

 clase, less frequently hornblende, and now and then clusters of epidote or chloritic masses 

 with calcite and magnetite, representing some completely metamorphosed mineral. 



Hornblendic Hiilleflintas. 



There is a not insignificant group of compact rocks much resembling halleflintas, but 

 with darker, greenish tones. They may be joined almost as appropriately to the halle- 

 flintas as to the hornblendic porphyroides. The name " hornblendic hallefliuta " may be 

 proposed for them. They are microgranitic or microfelsitic, but scarcely porphyritic, and 

 they differ from the halleflintas simply in the large number of particles of hornblende and 

 magnetite mingled with the quartz and felspar. 



V. — Conclusion. 



We have passed in brief review the various rocks of the drift of Central Ontario, 

 especially as found near Cobourg, and as we may suppose that farther investigation over 

 a wider range of country would add largely to the list of varieties, it is evident that our 

 drift presents a very interesting and extensive lithological collection. With the excep- 

 tion, however, of loose aggregations, such as clay, sand and gravel, and the very numerous 

 fossiliferous limestones of Lower Silurian origin, all the rocks examined are crystalline and 

 apparently of Archnean age. The almost complete absence of rocks of the later ArchiBau, 

 such as quartzites, slates and mica schists, may be noticed as a negative characteristic of 

 our drift. 



Among positive features may be remarked the large number of transitional forms, 

 intermediate varieties being found between the majority of the main groups of rock. 

 This is the less surprising, however, when we remember that rocks, unless composed of a 

 single substance, have in no sense the individuality of minerals ; for, by gradual additions 

 and subtractions, one often passes into another. This carries with it the disadvantage of 

 making clean-cut definitions difficult or impossible ; but also the advantage of bringing to 

 view the grand unity that underlies variety. 



Sec. iii, 1887. 8. 



