244 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



the gabbro, but, none the less, tbey have the appearance of having suffered 

 loss of volume through the solvent action of the magma. 



In some of the other sills the blocks are yet more numerous and many of 

 them are surrounded by shells of mixed material such as would result from the 

 solution of the quartzite in the basic magma. Since the blocks were suspended 

 in a magma of different density and since the product of solution was not 

 diffused away, the viscosity of the magma must have been high. Under these 

 special circumstances, it is not surprising that a limited amount of solution 

 was possible, even though the viscosity of the pure gabbro was relatively high. 

 On the one hand, the very strong contrast in the ionic composition of solvent 

 and substance dissolved, implies a specially great lowering of the melting point. 

 On the other hand, the original water of the sedimentary rock would facilitate 

 solution even at the comparatively low temperature of 1000° C. or less, at 

 which the nearly anhydrous gabbro became toughly viscous in cooling. 



Hybrid Bock. — A special instance was studied, optically and chemically, 

 In connection with material collected in one of the sills at a point on the main 

 Commission trail which is six miles up stream from the Boundary slash on the 

 west fork of the Yahk river and two miles north of the Boundary line. The 

 sill rock there forms low knobs on each side of the trail. Scores of gray, 

 angular quartzite blocks, surrounded by the gabbro, can be seen on the glaciated 

 ledges. 



One of these, measuring perhaps 100 cubic feet in volume, is enclosed in a 

 shell a foot or two thick, composed of the solutional mixture. The quartzite 

 has been completely recrystallized, with the development of large, poikilitic 

 quartzes, as in the case of the quartzite metamorphosed on the main contact 

 of the Moyie sill C (analysis in Col. 3, Table XIV.). Abundant, minute granules 

 of epidote were also developed. Recrystallized orthoclase (probably sodiferous) 

 and a little oligoclase are accessory constituents; no biotite could be found. 

 The original sediment must have been composed of nearly pure quartz and 

 -seems to have been far less feldspathic than the strata cut by the Moyie sills. 



A certain amount of osmotic action has taken place, for the quartzite is 

 shot through with narrow, greenish-black prisms of hornblende, 10 to 20 mm. 

 in length. This exotic hornblende has the optical properties of that in the 

 normal gabbro. It is specially abundant near the surface of the block which 

 is, however, sharply marked off from the shell of mixed material. Titanite and 

 apatite in notable amounts have also been introduced into the body of the 

 inclusion. 



The shell of mixed material consists of a coarse aggregate of deep green 

 hornblende in prisms 10 to 40 mm. long, and poikilitic quartz, which encloses 

 much granular epidote, titanite, apatite, a little ilmenite, and abundant, 

 minute prisms of the amphibole. No feldspar whatever is apparent in thin 

 section. 



An analysis of this mixed material (specimen No. 1164) gave Professor 

 Dittrich the following result: — 



