THICKI^ESS OF HEMLOCK FORMATION. 75 



cone, which in its turn, is dependent on the viscosity of the lava and the 

 presence of varying quantities of fragmental products. If we assume these 

 pre-Camhrian volcanic products to have had an initial dip of 15°, I believe 

 we are within limits for products consisting, as these do, of what was prob- 

 ably moderately viscous basalt and vast masses of fragmental material. 

 This estimate is based on the assumption that the volcanics here represented 

 were deposited for the most part upon the westward slope of a volcano, or 

 a series of volcanoes. This initial dip of 15° is then to be deducted from 

 the present dip, 75°, of the flows. Taking this into consideration, we get a 

 thickness of 23,000 feet for the volcanics. 



It is highly probable that the rocks have been subjected to close 

 folding, and for this reason also the apparent thickness would be much 

 greater than the true thickness. The schistose character of some of the 

 rocks shows clearly that they have been severely mashed, and this mashing 

 was probably produced in connection with folding. It is probable that this 

 possible maximum tliickness should be very materially reduced, possibly 

 to one-half or one-third of the amount. However, even the maximum 

 above calculated is probably paralleled by the vast masses of volcanic 

 material accumulated in certain volcanic areas, such as those of Hawaii or 

 Iceland. Greikie writes:^ 



The bottom of these Iceland Tertiary basalts is everywhere concealed under the 

 sea. Yet their visible portion shows them to be probably more than 3,000 meters in 

 thickness. 



An especial interest belongs to this Icelandic plateau because volcanic action is 

 still vigorous upon it at the present day. 



RELATIONS TO ADJACENT FORMATIONS. 



In the northern part of the Crystal Falls district the volcanics overlie 

 the quartzose dolomite formation known as the Randville dolomite. In 

 the central part of the district, through which the Deer River runs, as shown 

 in section G-H, PI. VI, outcrops are so scarce that it has been found 

 impossible to trace the boundaries of the formations with any degree of 

 accuracy. Consequently this part of the district is mapped as Pleistocene. 



From the few outcrops of slate, probably equivalent to the Mansfield 

 slate; which have been found in the Deer River area, it has been thought 



'The Tertiary basalt-plateaux of northwestern Europe, by Sir A. Geikie; Quart. Jour. Geol. 

 Soo. London, Vol. LII, 1896, p. 395. 



