534 GEORGE DAVIS LQUDERBACK 



Glaucophane and associated schists. — A striking feature of the 

 Dillard series is the irregular and sporadic occurrence of small bodies 

 of schists, certain types of which are characterized by the presence 

 of glaucophane. The variations in these schists, due to the presence 

 or absence or varying proportions of quartz, mica, glaucophane, 

 actinolite, and other amphiboles, garnets, epidote, chlorite, and 

 many other minerals, is very great, and it would lead too far, for the 

 present purpose, even briefly to characterize the different petro- 

 graphical types which are rather numerous in comparison with the 

 areal importance of the rocks. These schists are generally associated 

 with various more or less basic igneous rocks, and have been con- 

 sidered as products of contact metamorphism. The fact of prime 

 importance for the present consideration is that no outcrop of these 

 schists has yet been discovered in the Myrtle areas, while small 

 patches are of frequent occurrence in all areas of the Dillard. 



RELATIONS TO THE MYRTLE GROUP 



General relations. — While the rocks of the Myrtle group frequently 

 lie in contact with the different groups of igneous rocks just described, 

 careful search failed to bring to light any definite evidence of an 

 intrusive relationship. In every case the beds in contact with the 

 intrusives were apparently deposited upon the surface of the latter, 

 which must have been previously exposed by erosion. No traces of 

 tuff beds or lava flows were found anywhere in this group. 



General comparison with the Dillard. — If the areas of the Dillard 

 had been mapped separately from those of the Myrtle, probably the 

 most striking differential characteristics that would have been evi- 

 dent to a student of the folios are the practically entire lack of igneous 

 exposures within the lines of the Myrtle group, while the Dillard 

 areas are broken up and cut through again and again by dikes and 

 masses of the most variable shapes and sizes. The Myrtle area 

 most particularly studied by the writer is the type locality lying along 

 Myrtle Creek and extending a number of miles down through the 

 Riddles quadrangle. Here, as far as observed, igneous rocks are 

 entirely absent from the Myrtle area, except along its edges, where 

 they form the contact for a number of miles. Occasionally near the 

 edge of the syncline a small area of igneous rock appears within the 

 Myrtle boundaries and elongated in the direction of the axis of 



