788 



SCIENCE. 



[N. S. Vol. V. No. 125. 



N0TE8 FOB A COMPARISON BETWEEN TEE 



TERTIARY VOLCANIC SUCCESSION IN 



NORTHWESTERN EUROPE AND IN 



WESTERN AMERICA* 



Aftee alluding to the remarkably com- 

 plete record of volcanic action in Britain, 

 the lecturer proceeded to describe the most 

 extensive and best preserved of all the vol- 

 canic series which is referable to Ter- 

 tiary time and is remarkably developed 

 from the south of Antrim through the inner 

 Hebrides and Faroe islands to Iceland and 

 Greenland. A general sequence has been 

 noticed in the character of the erupted ma- 

 terial. The earliest lavas appear to have 

 been basalts, followed by andesites and, 

 lastly, dacites. The basalts form extensive 

 plateaus, each of which is built up of nearly 

 horizontal sheets. The intrusive rocks in- 

 cluded gabbros, ranging into peridotites 

 and other basic materials, granophyres, 

 granites, rhyolites and other acid rocks ; 

 the latest intrusions of all returning to the 

 original basaltic type. 



The earliest chapter in the Tertiary vol- 

 canic history of northwestern Europe brings 

 before us the gradual building up of exten- 

 sive plateaus of basalt which, in some 

 places, reach a thickness of more than three 

 thousand five hundred feet. These vast 

 outflows of lava appear to have issued 

 chiefly from fissures, like those of Iceland, 

 but with occasional vents, some of which 

 built up cones like those on the surface of 

 the Tertiary lava fields of western America. 



The general characters of the Scottish 

 basaltic plateaus were illustrated by a series 

 of lantern slides, and some examples were 

 also given of Icelandic fissures, particularly 

 of the great fissure which supplied the vast 

 lava floods of 1783. 



From the great antiquity of these Ter- 

 tiary lava plateaus they have undergone ex- 



*Abstract of a lecture by Sir Archibald Geikie given 

 before the Geological Society of "Washington on May 

 5th. 



tensive denudation and dissection, so that 

 their structure is laid bare along many 

 miles of picturesque coast line as well as on 

 the mountain ranges of the interior. Ex- 

 amples of the results of subaereal waste 

 were displayed on the screen. 



Continuing, the lecturer said that when 

 the basalts had accumulated to a great 

 thickness the magma appears to have found 

 easier passage between the strata at the 

 bottom of the volcanic series than upward 

 through fissures ; consequently, sheets or 

 sills are found intercalated among the Ju- 

 rassic rocks which form the base on which 

 the volcanic series rests, while the lower 

 portions of that series have been similarly 

 invaded. 



The next epoch in the volcanic history 

 reveals to us the uprise of an infinite num- 

 ber of successive intrusions of basic material 

 in sills, dikes and laccoliths at certain 

 points in the midst of the basalt plateaus. 

 On the whole, these masses consist of gab- 

 bros and dolerites, but they included also 

 some beautiful examples of peridotites. 

 There is no evidence that any of these in- 

 trusions reached the surface and appeared 

 there in the form of lavas. They seem ta 

 have risen by preference at certain points 

 of weakness, such as groups of vents. Ex- 

 amples of the relation between the gabbros 

 and vents of different sizes are particularly 

 observable in the islands of Skye and Mull. 



Illustrations of the scenery of the 

 gabbro bosses were shown by means of 

 slides, together with a portion of the crest 

 of the Cuillin Hills, to show the intri- 

 cate manner in which the material had 

 been injected. There was probably a long 

 interval after the cessation of the gabbra 

 intrusions. When the volcanic energy re- 

 sumed its activity the composition of the 

 underlying magma had completely changed. 

 Rocks of a thoroughly acid character were 

 now intruded. They took the form of 

 bosses, sills and dikes, but there is no 



