122 THE CRYSTAL FALLS IRON BEAKINU DISTRICT, 



some cases the separate portions of the hxva may have been originally 

 nearly globular, similar to the ones described by Platania. The ellip- 

 soidal basalts, however, are so common in the Crystal Falls district and 

 such globular basalts are so rare that this peculiar form is not considered 

 worthy of much consideration in the further discussion, the first two kinds 

 being chiefly the forms from which these were derived. 



The lava blocks rolling over one another as the lava stream advanced, 

 would lie with their axes in all positions, but pressure and the onward 

 mo^'ement of the flow would, in the lower portion of the stream at least, 

 be sure to produce from the blocks ellipsoidal bodies with their two longest 

 axes corresponding — the one to the direction of flow and the other to the 

 lateral extension of the stream. After the stream ceased to flow and the 

 lava solidified, there would be a gradation from the ellipsoidal into the non- 

 ellipsoidal portion of the flow. 



An aa stream, such as described and shown in fig. 10, when subjected 

 to great pressure subsequent to burial beneath thick deposits, would be 

 compacted by the breaking np of the jagged outer portions, which, falling 

 down, would fill the spaces between the blocks. This broken material 

 filling the spaces would be most exposed to movement and to the action of 

 percolating waters. It would consequently be very much altered, as in the 

 material described above (p. 119) by Ransome. Such alterations would 

 result in producing a matrix of exactly the same general composition as the 

 altered ellipsoids. It is the common case of metamorphic action producing 

 from rock masses of essentially the same chemical composition, but of 

 different character, similar end products. This brecciated character of jiarts 

 of this matrix is well shown in parts of PI. XI, and fig. B, PI. XXXIV. In 

 this case silica has been introduced, filling the spaces and marking out the 

 outlines of the fragments. "Where mashing has been excessive, the outlines 

 of the fragments are obliterated and the matrix rendered schistose. There 

 may even be a gradation from the schistose matrix into the altered basalt of 

 the ellipsoid, which at the center is massive. 



Let me recall the statement made on previous pages concerning the 

 distribution of the amygdaloidal cavities in the ellipsoids. This is one of 

 the characteristic features of the lavas. We have (1) amygdaloidal cavities 

 distributed about evenly throughout the ellipsoids, the cavities being some- 

 what smaller in the center than upon the periphery ; (2) the cavities are 



