o94 II. S. WASHINGTON — IGNEOUS COMPLEX OE MAGNET COVE 



The "Ridge." — A part from the surrounding shales and sandstones, there 

 is seen within the igneous area itself a horseshoe of what Williams calls 

 horn stone, or metamorphosed rock. This is either dark or almost white, 

 from rather coarse grained to aphanitic, often showing, apparently, traces 

 of stratification, and in most cases is evidently a highly metamorphosed 

 shale. This view is substantiated by microscopic study of the specimens 

 of these and of the shales at the contact in Diamond Jo quarry which I 

 collected. An important feature of the occurrence of these hornstones 

 is that they occupy the highest parts of the southern ridge, extending 

 also around to the northeast over the succession of hills north of the 

 basin. They are also found in thin strips on the northwest inside the 

 syenite, and bands of them are seen crossing the leucite-syenite south of 

 the main ridge. In every case these hornstones overlie, as far as can be 

 ascertained, the contiguous igneous rocks, which have in places broken 

 up through them. I could find no place along the " Ridge " where this 

 relation was absolutely proved, but a pretty thorough examination, to- 

 gether with consideration of the topographic relations and the presence 

 of dikes, left no doubt in my mind that the " hornstono " overlies the 

 igneous rocks. 



Arrangement of the igneous rocks. — A most important and suggestive 

 feature is the arrangement of the various igneous rocks. Although in 

 places hidden by alluvium and forest, yet the exposures are quite suffi- 

 cient to show the main features with clearness and certainty. The four 

 main types of abyssal rock are seen to occur in zones concentric about 

 a center and with the borders of the area. At the center, in the Cove, 

 we find the coarse grained, very basic " eleolite-mica-syenite (Cove type) " 

 with a central patch of soil full of masses of magnetite and decomposed 

 biotite. Williams suggests that this patch of " magnetite " is derived 

 from the decomposition of a rock more basic than the light Cove type 

 and approximating to the dark Cove type found just outside the main 

 area to the northeast. This seems very probable, and in the discussion 

 I have assumed it to be true. 



Surrounding this on the east, south, and west is a zone of somewhat 

 less basic and rather finer grained ijolite (eleolite-garnet-syenite, Ridge 

 type).* This is not seen on the north, except for a small patch, but we 

 can scarcely doubt that it encircles the other here also, though covered 

 by alluvium. It is also met with beneath the hill of halksinter in the 



* There is a serious error in the legend of Williams' map. The designations "Magnetite" and 

 " Eleolite-garnet-syenite (Kidge type) " opposite the second and third color squares should lie in- 

 terchanged. An area in the northeast which Williams colors as ijolite 1 found to he a foyaite, and 

 have so given it in my map. The mistake was undoubtedly the printer's. 



