Jo. A. Daly — Secondary Origin of Certain Granites. 189 



The Moyie Sill. — The acidification of the upper zone of the 

 gabbro being generally in a direct ratio to the strength of the 

 sill, the phenomenon is specially marked in the greatest of all 

 the intrusions. On account of its importance both in size and 

 character, this rock-body is called the "Moyie Sill," the name 

 referring to its situation on the Moyie River. A map and sec- 

 tioD of this sill are given m figs. 1 and 2, which illustrate one 

 of the fault-blocks so characteristic of this part of the Boundary 

 belt.* The sill is rather more than 2500 feet in thickness. It 

 follows the bedding of the Kitchener quartzite, which here dips 

 about sixty degrees to the eastward. The intrusive mass is 

 seen to be cut off at its northern end by a master-fault which 

 has dropped the Moyie argillite down into contact with the 



Fig. 2. Section of Moyie Sill, along line of the International Boundary. 



gabbro. This faulting is believed to have occurred after the 

 sill-intrusion. There is a complete lack of contact metamor- 

 phism in the argillite where it adjoins the gabbro. 



Since the Moyie sill, throughout the six miles of linear out- 

 crop studied, is in intrusive contact with the Kitchener quartz- 

 ite alone, the other sedimentary formations need not here be 

 described in detail. The Kitchener quartzite is, on the whole, 

 a homogeneous terrane. On a fresh fracture the rock is seen 

 to be a fine-grained, vitreous, light to darkish gray, well-bedded 

 but tough, metamorphic sandstone, splitting with some readi- 

 ness along the darker colored layers. The rusty color of the 

 joint-surfaces and bedding planes is due to the leaching out and 

 subsequent deposition of the iron contained in the pyrite, mag- 

 netite, etc., disseminated through the rock. 



Under the microscope the rock is always seen to be essen- 

 tially a fine-grained aggregate of interlocking quartz grains, 

 seldom showing any direct traces of their detrital origin. The 

 quartz mosaic is, in every thin section, shot through with 

 abundant crystals of biotite which is often developed in pheno- 

 cryst-like individuals occasionally as much as one centimeter 

 in diameter. Sericitic muscovite is seldom absent as an essen- 

 tial, and sometimes rivals the biotite in abundance. Only 



* All the line-drawings used in illustration of this paper have been made 

 for the most part by the aid of a typewriter, provided with a few special keys. 

 The machine permits of a great, saving of time in the preparation of the 

 manuscript drawings. Cf. this Journal, vol. xix, 1905, p. 227. 



