RELATIONS OF HEMLOCK FORMATIOK 77 



may be explained by supjjosing the volcanoes broke ont in the northern 

 part, while the Mansfield slate was still being deposited in the south. 

 Grradually, however, the volcanic activity spread toward the south, proba-- 

 bly following a fissure along the pre-Cambrian shore, and igneous materials 

 buried the Mansfield slate. Hence, while on the whole these volcanics are 

 younger than the Mansfield slates, some of the lower of them are con- 

 temporaneous with some of the upper Mansfield beds. The volcanics 

 invariably overlie the Randville dolomite, and are unquestionably of later 

 age than that formation. 



The Hemlock volcanics are overlain throughout their extent by the 

 Upper Huroniau series of graywackes and slates. Near the contact line 

 with the volcanics wherever the Huronian outcrops, or has been exposed 

 bv exploration, it has been found to be characterized by a line of magnetic 

 attraction. By means of magnetic observations the line of contact has been 

 traced, where owing to lack of exposures it would have been otherwise 

 impossible to connect the isolated outcrops. 



RBLATIOKS TO IKTRU8IVES. 



High ridges composed of dolerite are found extending in a general north- 

 west and southeast direction tln-ough the volcanics. That these masses 

 were forced up through the Hemlock formation is indicated by the folding 

 which they cause in certain places. Such rocks are unquestionably younger 

 than the volcanic series. There may be seen also on the map, in T. 44 N., 

 R. 32 W., a number of isolated knobs. These are also doleritic, and are 

 presumed to be, like the larger ridges, intrusive in the volcanics. 



The dolerites have in their turn been cut by acid dikes. These are 

 coarse micropegmatitic granites. Similar acid dikes have been foimd 

 cutting the surrounding volcanics. This set of acid dikes may be looked 

 upon as the youngest intrusive igneous rocks occurring in the Hemlock 

 volcanic formation. 



Cutting the volcanics are also basic dikes varying from fine to moder- 

 ately coarse grain. It is well known that during a volcanic epoch the out- 

 poin-ed lavas and clastic volcanic deposits are penetrated by dikes coming 

 from the same magma. Whether or not these dikes are of this origin, and 

 are hence contemporaneous with the later volcanics, or are of later age, and 



