140 THE VEKMILION IRON-BEARING DISTRICT. 



structure of the greenstone, one sees that these spots, which are also scattered 

 through the body of the rock, are frequently cross sections of irregular tubes 

 lying perpendicular to the surface of the flow. They are sections through 

 filled gas pores, the filling being known as amygdules. The amygdules 

 consist of chlorite, calcite, quartz, actinolite, and epidote. Rarely is any 

 one of the minei-als absolutely alone in the amygdule, though usually one or 

 the other will greatly predominate. The am5^gdules are filled with chlorite; 

 chlorite and quartz; calcite; chlorite and calcite; chlorite and epidote; 

 chlorite and actinolite; chlorite, calcite, and quartz; calcite and quartz; and 

 quartz alone. The materials constituting these amygdules are arranged 

 above in approximately the order of abundance. According to whether 

 the light or the dark minerals predominate, we get light-colored to white 

 granular areas on the one hand, grading to light-greenish to silky dark- 

 green areas on the other. The gas pores of these rocks owe their origin, 

 a's has already been hinted at, to the pressure of the gas in an originally 

 molten magma. When this magma reached a position where the pressure 

 was markedly diminished, the gas separated, segregated, and expanded, 

 and the magma became more scoriaceous on the surface; and the pores are 

 found to diminish in number and size as those portions of the original 

 molten magma that were under greater and greater pressure are reached. 

 A concomitant of the formation of the gas pores is the relativel}- rapid 

 cooling of the magma, producing rocks of a glassy nature or of very fine 

 grain. Both of these conditions commonly confront us in volcanic rocks 

 or, in other words, in rocks that have overflowed upon the surface. It is 

 true that amygdules have been observed in rocks which occur clearl}^ as 

 sills and dikes, and which therefore never actually reached the surface in a 

 molten condition. These cases are, however, relatively rare, and one can 

 readily see that the enormous reduction of pressure occasioned by the intru- 

 sion of the sills into their present places from much lower positions would 

 readily permit the expansion of the included gas. Moreover, in such cases 

 amygdules are far from numeroiis, showing that the pressure was dimin- 

 ished to such extent that relatively few pores were formed. In the case of 

 the amygdaloidal greenstones in the Vermilion district we observe the 

 following conditions : First, almost universally when amygdules are present 

 they occur in great quantity and are very commonly of large size ; second, 

 the amygdaloidal structure accompanies a fine-grained condition of the rock. 



