78 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



corres})()nd to the coarse dolerites, it is impossible to determine witli 

 certainty. Thev are presumed, however, to form an integral part of the 

 Hemlock volcanics, as no connection between the dikes and the unques- 

 tionably intrusive dolerites could be traced in the field. 



VOLCANIC ORIGIN. 



In spite of numerous occurrences of ancient volcanics which have 

 recently become known, the late Professor Dana makes the following- 

 statement:^ 



It is not yet certain that a volcauo ever existed ou the continent of North 

 America before the Cretaceous period; for the published facts relating to supposed 

 or alleged volcanic eruptions in tbe course of the Paleozoic ages are as well explained 

 on tbe supposition of outHows from fissures and tufa ejections under submarine 

 conditions; and none of the accounts present evideuce of the former existence of 

 a volcanic cone, that is, of an elevation i^ericentric in structure made of igneous 

 ejections. 



The presence in the Hemlock formation of a quantity of pyroclastics, 

 great in proportion to the solid lavas, and the absence of any great sheets 

 of lava, so important a product of g'reat fissure eruptions, seem to point to 

 the derivation of the Hemlock rocks from a volcano or volcanoes situated 

 near the border of the contemporaneous Huronian sea, rather than from a 

 simple fissure. While some of the eruptives may have been submarine, 

 the occurrence of large quantities of clearly subaerial deposits shows that the 

 eruptives were largely on the land. Thus it appears that neither a fissure 

 flow nor a submarine volcano will wholly explain the Hemlock formation. 

 However, it is highly probable that this volcanic outburst, which piled 

 masses of A'olcanic material upon the land, was accompanied, as have been 

 all or nearly all the great outbursts of recent times, by submarine lava 

 flows and tuff" ejections. No such clear evidence of the presence of a Pale- 

 ozoic or pre-Paleozoic volcano on the North American continent has been 

 adduced as that given by the English geologists for certain volcanoes 

 in the British Isles. But while the presence of a central cone with peri- 

 centric arrangement in the Hemlock district is not conclusively proven, 

 the presumption in favor of such a cone or cones having existed is certainly 

 strong. 



'Manual of Geology, by J. D. Dana: 4tli ed., 1895, p. 938. 



