178 



SIDNEY POWERS 



began crystallizing." The syenite has been somewhat altered by 

 mineralizing vapors from the magma. 



The sapphire-bearing Yogo dike also contains inclusions.' 

 This mica-trap dike cuts Madison limestone. It is from 3 to 6 

 feet wide and is vertical. The dike walls are rough, but not espe- 

 cially irregular, and have been slightly indurated by the intrusion. 

 In some places the upward termination is seen, as shown in Fig. 3. 

 The fragments are angular and consist of limestone and shale from 

 the Madison and underlying terranes. In the main excavations 

 a similar breccia is shown at the surface, but the size and number 

 of the fragments decrease with depth. 



The fragmental material has evi- 

 dently been shattered from the 

 fissure walls and floated upward 

 as the molten rock rose in the 

 fissure. The Yogo dike is the only 

 case presented in this paper where 

 the inclusions are proven to have 

 floated up to the top of the dike. 

 From the diminution in the number 

 of inclusions with depth, it is evi- 

 dent that if the dike were exposed 

 only at a lower level, few if any in- 

 clusions would be shown. On the 

 other hand, inclusions are rarely 

 present at the upward termination of dikes, or, if present, they 

 have not been mentioned in the descriptions.^ 



Syracuse, New York: A few inclusion-bearing peridotite dikes 

 at Syracuse, New York, have been described by several writers. 

 The country rock is of Salina (Silurian) age. The inclusions are 

 largely of Paleozoic rocks, but some are of pre-Cambrian gneisses. 

 The latter are usually more rounded than the former, the result 

 of the attrition involved in their upward journey.^ In one dike 

 ' W. H. Weed, U.S. Geol. Siirv., 20th Ann. Rept., Part 3, p. 455. 



2 Suess in Das Antlitz der Erie, Vol. Ill, Part 2, p. 658, describes several dikes the 

 tops of which are exposed, but the above is the only one in which inclusions are men- 

 tioned. 



3 C. H. Smyth, Jr., Amer. Jour. Sci. (4), XIV (1902), 26. 



Fig. 3. — Section of upper limit 

 of sapphire-bearing dike, wall of 

 Yogo Canyon, Montana. The in- 

 clusions are limestone and shale 

 fragments from the fissure walls, 

 which have floated up to the top of 

 the dike. (After Weed.) 



