Some Dykes cutting Zaurentian Series. 487" 



smaller and less regular ones. The magnetite occurs- 

 in octahedrons, irregular grains, and in skeleton crystals- 

 or dendritic forms. A number of octahedrons are often 

 seen joined together into strings, having branches at 

 right angles. The porphyritic plagioclase, Fig. 2 (a) } . 

 usually has the lath-shape and an average length of less 

 than 1 mm. Phenocrysts of this mineral are more abun- 

 dant than those of pyroxene, with which they are often 

 intergrown — the pyroxene being the older, and having- 

 served as a point of attachment for the feldspar when 

 it began to crystallize. The porphyritic crystals of" 

 monoclinic pyroxene are light-brown to colourless, and. 

 frequently have a somewhat elongated form, with rough 

 edges. Some of the largest are 1*8 mm. in length. An 

 irregular parting perpendicular to the longest axis of the 

 crystal is present. Ehombic pyroxene occurs sparingly 

 in some of the sections. Its phenocrysts are smaller than 

 those of the monoclinic, and generally have a somewhat 

 regular octagonal outline. Patches or irregular grains of' 

 chloritic material are present in all of the thin sections 

 examined. 



Thin sections were examined from some larger dykes 

 in the same vicinity. These were found to possess the 

 general characters of the rocks just described. Pyroxene,, 

 however, was not observed among the phenocrysts, and 

 olivine appeared to be absent. A flow structure is often 

 seen in the sections from the narrow dyke. This struc- 

 ture is illustrated by the arrangement of the feldspar 

 phenocrysts and the constituents of the groundmass 

 around the earlier-formed individuals. This dyke rock,, 

 which may be classed as a porphyrite, appears to resemble- 

 the members of the effusive group more closely than it- 

 does those of the plutonic. Effusive masses, however, are- 

 not known to overlie the Laurentian in any part of the- 

 district. 



