922 A TREATISE ON METAMORPHISM. 



RELATIONS OF IGNEOUS ROCKS TO STRATIGRAPHY. 



Before considering the relations of igneous rocks to stratigraphy it is 

 necessary to recall the forms which they usually take and the manner in 

 which they may be expected to die out. The intrusives are ordinarily 

 known as batholiths, bosses, laccoliths, sills, and dikes. The extrusives 

 are either lavas or tuffs. Frequently dikes radiate from a batholithic or 

 laccolithic mass, become less and less abundant in passing away from the 

 igneous mass, and finally disappear. Not infrequently the intrusives and 

 extrusives are closely associated, intrusives being in one part of a region 

 or district and equivalent extrusives in another part. But whether the 

 rocks are intrusive or extrusive, if the quantity of igneous material be 

 large and in various forms the igneous rocks usually do not die out at 

 once, but gradually. Ordinarily the distance required for their disappear- 

 ance is considerable, but in certain cases there are exceptions to this. 



For the most part the ordinary masses of volcanic rocks do not 

 profoundly affect the beds with which they are associated. On the other 

 hand, large intrusive masses may metamorphose surrounding sediments 

 for miles. 



As a consequence of the gradual disappearance of eruptives under 

 ordinary circumstances, the abundance of eruptives in one horizon and 

 their absence in an adjacent horizon indicate that a structural break 

 probably exists between the two, or that a fault has displaced the strata. 

 The absence of phenomena which indicate faulting renders the first 

 alternative probable. The sudden disappearance of eruptives is a particu- 

 larly valuable criterion in separating adjacent series in cases in which 

 parallel secondary structures have been induced in both. These secondary 

 structures may have obliterated or nearly obliterated the original bedding, 

 so that there may be none of the ordinal'}- evidences of structural breaks; 

 but if a set of dikes passes to a certain horizon and is suddenly cut off, 

 this is very suggestive of an erosion interval between the two series. In 

 cases of this" kind the eruptives themselves are likely to have been 

 extensively altered by the forces which produced the secondary structures 

 in the sedimentary rocks: they may have been changed from their original 

 condition to schists, and then may be shown to be dikes or intrusives by 

 their structural behavior, by microscopical examination, or by chemical 

 analysis. 



