3IO PRINCIPLES OF STRATIGRAPHY 



ments. Such contacts may be sharp, where the surface of the 

 igneous mass has been swept clean before the deposition of the 

 sediments, as in the case of the pre-Cambric granite floor of the 

 Manitou region in Colorado (Crosby-6), upon which abruptly 

 appear the clean-washed sands of the basal Palseozoic of that 

 region (Fig. 52), or it may be a transition contact where the old 

 igneous mass has been much decomposed, while the subsequent 

 sediment is made up of the partly reworked upper layers of the 

 old regolith. An example of this is found in the contact of the 

 Lake Superior sandstone of the Marquette region with the altered 

 peridotites of pre-Cambric age, where it is often impossible to 



I inch = lOffet. 



Fig. 52. Irregularities in sedimentary contact, of basal Palaeozoic sandstones 

 on pre-Cambric granite in Williams Canyon, Colorado. The gen- 

 eral character of the contact is a sharp and, for the most part, 

 very smooth one. (After Crosby.) The granite mass is part of 

 an abyssolith. 



determine the precise line of contact. Great difference in age may 

 exist between these two. 



For such an ancient igneous mass, which has been uncovered by 

 erosion, and subsequently covered by sediments, with which it is in 

 sedimentary contact, the name Abyssolyth is proposed. Such an 

 abyssolyth is seen in the granite mass of Pikes Peak, the contact 

 of which, with the Palaeozoics, is a sedimentary one. (Fig. 52.) 



Contacts of hypabyssal or injected masses. 



Dikes, sills, laccoliths, etc., are younger than the strata which 

 they cut, or which they have metamorphosed. Their proximal age 

 limit is, however, not determined by the strata in which they occur, 

 unless some of these strata should contain erosion fragments of 

 the intrusives, when the latter must be considered as very much 

 older than the strata containing such fragments, and which are in 

 sedimentary contact with the igneous masses. 



