440 



SCIENCE. 



[N. S. Vol. IX. No. 221. 



work on the Ancient Volcanoes of Great 

 Britain, in comparing the volcanic phenom- 

 ena of the Icelandic eruptions with those 

 exhibited by the basalt plateaus of the 

 British Isles, remarks : " It is, therefore, to 

 the Icelandic types of fissure eruption, and 

 not to great central composite cones, like 

 Vesuvius or iEtna, that we must look for 

 the modern analogies that would best serve 

 as commentary and explanation for the 

 latest chapter in the long volcanic history 

 of the British Isles." 



In comparing volcanic areas of Iceland 

 with the phenomena exhibited in the Ab- 

 saroka Eange there is one striking diifer- 

 ence to be noted. In the former the ex- 

 travasated molten magma consists largely 

 of basaltic flows, while in the latter one is 

 constantly impressed by the enormous 

 amount of brecciated rock emitted. It is 

 estimated that four-fifths of these extrusive 

 rocks which make up the range consist of 

 coarse and fine breccias, silts and related 

 ejectamenta. Dead Indian Peak, one of 

 the dominating points of the range, rises 

 more than 6,000 feet above the valley, pre- 

 senting layers of breccia which in the ag- 

 gregate measure nearly one mile in thick- 

 ness. It is a very conservative estimate to 

 place the volume of breccia at one-half mile 

 in thickness over the entire region under 

 discussion, which, it should be remembered, 

 embraces not much less than 4,000 square 

 miles. This only allows for erosion an 

 amount equal to the highest plateau sum- 

 mit, but it is sufficient to give one an idea 

 of its vast bulk. That the denudation from 

 the top of the existing plateau was very 

 considerable is unquestioned, but there 

 exist, I think, no I'eliable data upon which 

 to base even an approximate estimate of 

 the amount. Possibly the country was at 

 one time covered with a mantle of basalt, 

 which, withstanding erosion, would, of 

 course, pi-otect the friable volcanic material 

 throughout a long period. 



It is evident that the granular rocks re- 

 quired for their uniform crystallization an 

 overlyingload of greater or less depth. For 

 my own part, I am more or less skeptical as 

 to the need of an immense thickness of over- 

 lying material to develop such uniform con- 

 solidation as is generally supposed to be 

 necessary to produce the so-called plutonic 

 rocks. At Needle Mountain the medium- 

 grained granular diorite for the entire 4,000 

 feet of rock face is apparently the same 

 throughout, whereas only a short distance 

 from the mass and at a lower level small 

 bodies of rock in cooling have developed a 

 characteristic andesitic structure. 



It must be borne in mind that all this 

 material, of varied mineral composition, 

 grouped together under the designation of 

 breccias, was congealed and crystallized be- 

 fore it was hurled out by explosive action. 

 This means stupendous crushing and 

 crunching of the mass as it was forced up- 

 ward, and disturbances of the first magni- 

 tude, which must have had their origin in 

 great crustal movements. Whence came 

 this enormous mass of brecciated rock ? 

 Twice during the long period of their 

 eruptions these breccias had been in- 

 vaded by enormous bodies of granular rock 

 which had elevated the entire Absaroka 

 Eange, an elevation that was phenomenal 

 in its nature and formed a part of the great 

 series of orogenic movement which uplifted 

 the northern Cordillera. This uplift was 

 closely related to the post-Laramie move- 

 ment, which was one of the most profound 

 and far-reaching orogenic disturbances any- 

 where recognized by geologists. 



Throughout this address the large in- 

 dividual protrusions into the breccia have 

 been alluded to as stocks, but I regard 

 them as the more elevated portions of a 

 great complex of crystalline rocks under- 

 lying at least a large part of this region of 

 country. Where the underlying molten 

 magma was subjected to the severest pres- 



