February 1, 1887.] 



♦ KNO^A^LEDGE 



85 



no means the earliest, though they are the earliest of which 

 we have clear pala^nntological records. The eruptive powers 

 of the earth during the whole of the jniuiary age must have 

 been enormously greater than during the secondary age; 

 but the tremendous results of the action of primary vol- 

 canoes have been removed millions of years since by sub- 

 aerial denudation. The eruptions of the secondary age, 

 again, must have been far more tremendous than those of 

 the tertiary age ; but even of those we have but few records 

 left. In fact, we need only consider the condition in which 

 the products of tertiary volcanic action are now left for our 

 study to understand how utterly unsatisfictory and imper- 

 fect must be the evidence e.vtant in regard to the volcanoes 

 of the secondary and primary periods. 



In our own isles we find very remarkable evidence of 

 tertiaiy volcanic action, yet is that evidence at least as 

 impressive in what it suggests but leaves unsaid as in what 

 it actually reveals. The isles of JMuU and Skye may be 

 regarded as the wrecks of enormous volcanic mountiiins of 

 the tertiary age, probably more than a million years old. 

 Each was about 30 miles in diameter, and each about 

 13,000 feet, probablj', in height. Each continued for many 

 thousands of years to eject froui time to time enormous 

 masses of various kinds of lava, as well as scoriaj and lapilli, 

 such as Vesuvius and Etna eject, but in much greater quan- 

 tities. Of the size of those great volcanoes, and of the 

 tremendous energy of their eruptive powers, we have very 

 clear evidence. But the evidence wanting speaks in its 

 silence more impressively by far than the evidence still re- 

 maining. Of the vast mass of either volcanic mountain but 

 the merest fragments are now left. Low moinitains, little 

 more than hills, certainly not moi'e than a foui'th the 

 height of the former mountain masses, show where these 

 active volcanoes once stood. All the rest has been worn 

 and washed away by rain and wind, and snow and storm. 

 8o far back as the memory of man runs, Skye and AIull 

 were as they are now, so slowly, though so steadfastly, do 

 the denuding, like the upheaving, forces of the earth do 

 their work. Yet the clear signs remain that masses not 

 merely larger but many times larger than the whole present 

 mass of either island above the sea-level have been destroyed. 

 Probably seveu-eighths of the former material above the sea- 

 level has been carried away, and is now beneath the waves 

 of ocean. If all this has been done since the tertiary age, 

 what chance can there be of our detecting more than the 

 merest fragments of the volcanic products of the much- 

 longer secondary and primary periods, even though these 

 volcanic products were ejected on a much grander scale t In 

 yet another, and, indeed, a much more striking way, have 

 the interior forces of the earth in the tertiary' period left 

 records of their energy. In Montana and Wyoming a tract 

 as large as France and Germany together was covered with 

 basaltic lava to a depth of from six or seven hundred to 

 three or four thousand feet. In the region now occupied 

 by the British Islands, again, from Antrim to Mull and 

 Skye and far northwards, even perhaps continuously to the 

 Faroe Islands in one direction and to Iceland in another, 

 similar masses wei'e poured forth. In the Giant's C'auseway 

 this basaltic lava has a depth not exceeding anywhere 8(10 

 feet. But among the islands opposite the western shores of 

 Scotland there aie places where the lava shows a depth of 

 more than 3,000 feet. In the Faroe Isles a deptli four times 

 as great is indicated. The very circumstance, however, 

 that we can determine the present thickness of these 

 immense lava beds, compared with which all that has ever 

 been poured out by Etna and Vesuvius, or even by Ilecla, is 

 but as a lake compared with the sei, tells us that the original 

 thickness must have been much greatei', and that the 

 quantity whicli has worn and crumbled away under the 



action of the denuding forces of air and water must prob- 

 ably have equalled, if it did not exceed, even those immense 

 masses which still remain. On the north-eastern shores of 

 Ireland and along tl)e south-eastern parts of Scotland we 

 see wheie the ocean has cut its way into the basaltic lava, 

 easting down the columnar blocks into which the lava had 

 formed itself as it .shrank. The regularity of the hexagonal 

 form in most parts of the causeway is as directly a result of 

 physical law, it may be mentioned, as the hexagonal form of 

 the honeycomb. The basalt simply gave way, in shrinking, 

 where resistance to cleavage was most easily overcome ; and 

 so, where uniform in material, produced uniformly hexagonal 

 blocks, just as the bee working so as to )ise up the least 

 amount of wax produces hexagonal cells, unconscious of the 

 fact that it is working out a pretty mathematical pi-oblem. 

 While the slow wearing away of the basaltic masses by the 

 sea waves went on, the whole upjjer surface was undergoing 

 steady denudation. Frost and thaw, snow and rain, the 

 drying action of the sun, followed by the work of the wind 

 in removing the dust into which the rock has been always 

 crumbling at its surface — all these processes, scarcely affect- 

 ing the aspect of the region appreciably in many centuries, 

 must have removed a large proportion of its original mass 

 during the hundreds of thousands of years which have 

 elapsed since it was extruded. In IMontana and Wyoming, 

 indeed, we find comparative youth ; but the way in which 

 such youth is indicated shows what an extreme old age it 

 signities as compared with the periods l)y which we measure 

 history. For the rivers and torrents, which in past ages 

 have worked those channels into the rock which ai'e called 

 canons, have not as yet worked their way down more than 

 seven or eight hundred feet, many having run dry after 

 doing that portion of the work of channel carving. When 

 we see how slowly even the fierce rapids and mighty falls of 

 Niagara cut awaj' the rocks between and over which they 

 rush, we can infer the vastness of the periods of which the 

 great canons of the north-western States give evidence. 



All the work which Etna and Vesuvius have done since 

 they first existed as volcanic outlets belongs but to the 

 closing and comparatively restful portion of the history of 

 the great mountain system of Southern Eui-ope. They are 

 but flank outlets, no more to be compared with the original 

 fissures through which the core of the Alps was extruded 

 millions of years ago, than the small side craters on their 

 own slopes with the chief vents by which Etna and 

 Vesuvius aflbrd the intern.al forces of the earth relief. For 

 hundreds of thousands of years those original fissures poured 

 forth molten masses. For still vaster periods the region of 

 fissure-ejection sank beneath a wide-spreading sea : foot by 

 foot, yard by yard, mile by mile, the trough sank and 

 sedimentary matter was deposited in it (so that the sea 

 remained ever shallow), till strata ten miles deep had been 

 formed. Then, during hundreds of thou.sands of years, the 

 sea-floor, shrinking all round, shouldered up the great core 

 of deposited matter, bending, grinding, and contorting it till, 

 witli the tremendous heat generated in the process, its whole 

 character was altered. Denuding forces carved and chiselled 

 out of the heterogeneous material the nuunitain peaks which 

 now alone remain of the upheaved dome-shaped masses. 

 Along the chief lines of original disturbance there is now 

 no longer volcanic activity. On the flanks even volcanic 

 action has for the most part died out ; but far away from 

 the core, flanking the flanks as it were of the great 

 mountain ranges, we find a few disturbed regions, whose 

 outlets we see in Etna, Vesuvius, Stromboli, and the rest. 

 An outburst like the one lately in i>rogress, representing as 

 it does but a remnant ot a remnant of the pa.'^t vulcanian 

 energies of South Europe, atte:i'.s most strikingly the over- 

 whelming might of those energies in the past. — Times. 



