VOLCANIC ORIGIN OF HEMLOCK FORMATION. 79 



An attempt was made to locate tlie vent or vents from which the 

 material was derived, but no evidence could be foimd, unless we consider 

 the vents to have been where the accumulations were the greatest. The 

 coarse-grained rocks which were first supposed to represent • the plugs of 

 ancient volcanoes, on careful and detailed examination appear to be later 

 intrusives, or else are indeterminate. 



CLASSIFICATION. 



The general character and distribution of the Hemlock formation hav- 

 ing been given, we may now proceed to a petrographical consideration of 

 the rocks comprising it. This will be given in more detail than for the 

 other rocks of the Michigamme district because this great pre-Cambrian 

 volcanic formation possesses peculiar interest. 



The rocks of the Hemlock formation are chiefly of direct igneous 

 origin. Some interleaved sedimentary rocks occur, which, however, with a 

 single exception are composed of fragments of the igneous rocks. For the 

 sake of easy reference, the usual classification into igneous and sedimentary 

 rocks will be used. The massive igneous rocks are subdivided according to 

 chemical composition into acid and basic rocks. The acid rocks include 

 rhyolite-porphyries,^ aporhyolite-porphyries, and acid pyroclastics. The 

 basic rocks include altered nonporphyritic basalts, porphyi'itic basalts, and 

 variolite and basic pyroclastics. The sedimentary rocks are divided into 

 the volcanic sedimentaries and the nonvolcanic sedimentaries or normal 

 sedimentaries. The first include tuff's and ash beds — the teolian deposits, 

 and volcanic conglomerates — subaqueous dejiosits. The normal sedimen- 

 taries are represented by slates and limestones. Various schists are locally 

 produced from these immerous kinds of rocks through metasomatic changes 

 and dynamo-metamorphic action. Many of these schists resemble one 

 another very closely, though, as will be seen later, they are derived from 

 both the massive rocks and from the elastics. These have been described 

 in connection with the rocks from which they have been derived. 



'According to a late ruliug of the Director of the United States Geological Survey, based on the 

 recommendation of a committee on nomenclature for geologic folios, "porphyry" is to be used only 

 ■with a textural significance. Hence "quartz-porphyry," according to this ruling should uo longer be 

 used as a rock name. The rhyolite-porphyries here described are what have been known as normal 

 quartz-porphyries. 



