NATURE 



\_Nov. 4, iSfc'o 



them, presented difficulties wtiich in the light of modern 

 volcanic action remained insoluble. The wonderfully 

 persistent course and horizontality of the basalts with the 

 absence or paucity of interstratified tuffs, and the want of 

 any satisfactory evidence of the thickening and uprise of 

 the basalts towards what might be supposed to be the 

 vents of eruption were problems which again and again I 

 attempted vainly to solve. Nor so long as the incubus of 

 " cones and craters " lies upon one's mind does the ques- 

 tion admit of an answer. A recent journey in Western 

 America has at last lifted the mist from my geological 

 vision. Having travelled for many leagues over some 

 of the lava-fields of the Pacific slope, I have been 

 enabled to realise the conditions of volcanism described 

 by Richthofen and, without acquiescing in all his theore- 

 tical conclusions, to judge of the reality of the distinction 

 which he rightly drew between "massive eruptions" 

 and ordinary volcanoes with cones and craters. Never 

 shall I forget an afternoon in the autumn of last year 

 upon the great Snake River lava desert of Idaho. It 

 was the last day of a journey of several hundred miles 

 through the volcanic region of the Yellowstone and 

 Madison. We had been riding for two days over 

 fields of basalt, level as lake bottoms, among the valleys, 

 and on the morning of the last day, after an inter- 

 view with an armed party of Indians (it was only a few 

 days before the disastrous expedition of IVIajor Thorn- 

 burgh, and the surrounding tribes were said to be already 

 in a ferment), we emerged from the mountains upon the 

 great sea of black lava which seems to stretch inimitably 

 westwards. With minds keenly excited by the incidents 

 of the journey, we rode for hours by the side of that appa- 

 rently boundless plain. Here and there a trachytic spur 

 projected from the hills, succeeded now and then by a 

 valley up which the black flood of lava would stretch 

 iiway into the high grounds. It was as if the great piain 

 had been filled with molten rock which had kept its level 

 and wound in and out along the bays and promontories 

 of the mountain-slopes as a sheet of water would have 

 done. Copious sprmgs and streams which issue from the 

 mountains are soon lost under the arid basalt. The 

 Snake River itself, however, has cut 'out a deep gorge 

 through the basalt down into the trachytic lavas under- 

 neath, but winds through the desert without watering 

 it. The precipitous walls of the canon show that the 

 plain is covered by a succession of parallel sheets of 

 basalt to a depth of several hundred feet. Here and 

 there, I was told, streams thai have crossed from the 

 hills and have flowed underneath the lava-desert issue 

 at the base of the caiion-walls, and swell the Snake 

 River on its way to the Pacific. The resemblance of 

 the horizontal basalt-sheets of this region to those with 

 which I was familiar at home brought again vividly 

 before my mind the old problem of our Miocene dykes 

 and Richthofen' s rejected type of "massive" or fissure 

 eruptions. I looked round in vain for any central cone 

 from which this great sea of basalt could have flowed. 

 It assuredly had not come from the adjacent mountains, 

 which consisted of older and very different lavas round 

 the worn flanks of which the basalt had eddied. A few 

 solitary cinder cones rose at wide intervals from the 

 basalt plain, as piles of scoria: sometimes do from the 

 vapour vents on t'le surface of a Vesuvian lava-stream. 



and were as unequivocally of secondary origin. Riding 

 hour after hour among these arid wastes, I became con- 

 vinced that all volcanic phenomena are not to be explained 

 by the ordinary conception of volcanoes, but that there is 

 another and grander type of volcanic action, where, in- 

 stead of issuing from separate vents and piling up cones 

 of lava and ashes around them, the molten rock has risen 

 in fissures, sometimes accompanied by the discharge of 

 little or no fragmentary material, and has welled forth so 

 as to flood the lower ground with successive horizontal 

 sheets of basalt. Recent renewed e.xamination of the 

 basalt-plateaux ■ and associated dykes in the west of 

 Scotland has assured me that this view of their origin 

 and connection, which first suggested itself to my mind 

 on the lava-plains of Idaho, furnishes the true key to 

 their history. 



The date of these lava-floods of the Snake River is in 

 a geological sense quite recent. They have been poured 

 over the bottoms of the present valleys, sealing up be- 

 neath their sheets of solid stone river-beds and lake-floors 

 with their layers of gravel and silt. The surface of the 

 lava is in many places black and bare as if it had cooled 

 only a short while ago. Yet there has been time for the 

 excavation of the Snake River cafion to a depth of 700 

 feet through the basalt-floor of the plain. In so arid a 

 climate, however, the denudation of this floor must be 

 extremely slow. Much of the plain is a verdureless waste 

 of loose sand and dust which has gathered into shifting 

 dunes. Save in the gorges laid open by the main river 

 and some of its tributaries hardly any sections have yet 

 been cut into the volcanic floor. Dykes and other pro- 

 trusions of basalt occur on the surrounding hills, but the 

 chief fissures or vents of emission arc still no doubt buried 

 beneath the lava that escaped from them. 



In North-Western Europe, however, the basalt-sheets 

 were erupted as far back as Miocene times. Since then, 

 exposed to many vicissitudes of geological history — sub- 

 terranean movement and changes of climate, with the 

 whole epigene army of destructive agencies, air, rain, 

 frost, streams, glaciers, and ice-sheets — the volcanic 

 plateaux, trenched by valleys two or three thousand feet 

 deep and a mile or more in breadth, and stripped bodily 

 off many a square mile of ground over which they once 

 spread, have been so scarped and cleft that their very 

 roots have been laid bare. Viewed in the light of the 

 much younger basalts of the Western Territories of North J 

 America, their history becomes at last intelligible and I 

 more than ever interesting. We are no longer under the 

 supposed necessity of finding volcanic cones vast enough 

 to have poured forth such wide-spread floods of basalt. 

 The sources of the molten rock are to be sought in those 

 innumerable dykes which run across Britain from sea to 

 sea, and which in this view of their relations at once fall ^ 

 into their place in the volcanic history of the time. I 



No more stupendous series of volcanic p'nenomena ' 

 has yet been discovered in any part of the globe. We 

 are first presented with the fact that the crust of the 

 earth over an area which in the British Islands alone 

 amounted to probably not less than 100,000 square miles, 

 but which was only part of the far more extensive region 

 that included the Faroe Islands and Iceland, was rent by 

 innumerable fissures in a prevalent east and west or south- 

 east and north-west direction. These fissures, whether due 



