36 



ANXAL8 NEW YORK ACADE2IY OF SCIENCES 



Igneous Structures 



The most striking thing in connection with the structure is the re- 

 markable uniformity of the sills and their close resemblance on that ac- 

 count, after weathering, to the fragmental beds with which they are 

 associated. The small amount of metamorphosing influence that they 

 seem to have had, also adds to the difficulty. In some cases, however, a 

 transgressive intrusion has disturbed the adjacent beds a great deal in a 

 mechanical way. 



In the average case, it is judged that the intruded magma has neither 

 penetrated the materials of the adjacent beds to a noticeable amount, nor 



Fig. 11. — Shales and ash beds cut Tjy a large irregular dike and sill 



The dike is shown at the location of the standing figure and the sill extends upward 

 to the left between the plainly bedded layers. Both the dike and the sill are crowded 

 with fragmental material to an extent that makes the intrusive have more the appear- 

 ance of a volcanic fragmental than a true intrusive. 



has it absorbed or incorporated a great deal of such material. But in a 

 few cases where structural relations were indisputable it was equally 

 clear that the intrusives, both transgressive and concordant, were liter- 

 ally choked with foreign fragmental matter, making them resemble the 

 real tuffs so closely that it is doubtful whether the difference would have 

 been detected except for the clearness of the intrusive relation. Such 

 occurrences suggest that there may well l^e many other apparently frag- 

 mental interbeds that are in reality fragment-clogged intrusives. On ac- 



