DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. Ill 



Between 160 and 170 separate cones have been counted on this area, most of them quite 

 small, mere low mounds of scoriae, though a few reach a height of 700 or 800 feet, with a 

 diameter of a mile. From three to seven or eight may be found in a row, as if springing 

 from a single line of fissure. But generally the grouping is quite irregular."" My friend 

 Captain C. E. Dutton, from whose admirable memoir these details are quoted, remarks 

 further that among the Utah plateaux no trace of a cone is to be found at or near some 

 of the most recent basalt-fields, and that the most extensive outpours are most frequently 

 without cones. " The lavas," he adds, " appear to have reached the surface and over- 

 flowed like water from a spring, spreading out immediately and deluging a broad surface 

 around the orifice." t The deep gorges cut by the rivers through these thick accumulations 

 of horizontal or nearly horizontal basalts, have here and there revealed the parallel dykes 

 that traverse the rocks, and in at least one case have shown the dyke running for half a 

 mile up a cliff and actually communicating with a crater of scoriae at the top. J Again, 

 in New Mexico, Captain Dutton noticed vast tracts of younger basalt, about which " a 

 striking fact is the entire absence of all distinguishable traces of the vents from which 

 they came. Some of them, however, indicate unmistakably their sources in small 

 depressed cones of very flat profiles. No fragmental ejecta (scoriae, lapilli, &c.) have been 

 found in connection with these young eruptions." § Such I believe to have been the 

 general conditions under which the basalts of the Tertiary plateaux of the British Isles 

 were also erupted. 



§ 4. Intrusive Sheets or Sills of the Plateaux. 



There is one further part of the structure of the basalt-plateaux of which some 

 account must now be given. In a former paper, I have shown that at different parts of 

 the basalt series, but especially at their base and among the stratified rocks underneath 

 them, sheets of basalt and dolerite occur which, though lying parallel with the strati- 

 fication of the volcanic series, are not truly bedded masses, but are intrusive sills, 

 and therefore of younger date than the rocks between which they lie. || The non- 

 recognition of their intrusive and subsequent nature led to these sheets being 

 regarded as proofs of the intercalation of volcanic beds in the Jurassic series of western 

 Scotland. There is, however, not the least trace of the true interstratification of a 

 volcanic band in any part of that series, every apparent example being due to the 

 way in which intrusive sheets simulate the characters of contemporaneous flows. 



If such sheets had been met with only at one or two localities, we might regard them 

 as due to some mere local accident of structure in the overlying crust through which the 

 erupted material had to make its way. But when we find them everywhere, from the 



* C. E. Dutton, "Tertiary History of the Grand Canon District," U.S. Geol. Swrvey (1882), p. 104. 

 t C. E. Dutton, " Geology of the High Plateaux of Utah," U.S. Geol. Survey of the Rocky Mountain Region (1880), 

 pp. 198, 200. See also pp. 232, 234, 276 of the same Monograph for additional examples. 

 J Tertiary History of the Grand Canon, &c, p. 95. 

 § Nature, xxxi. (1884) p. 89. 

 || Quart. Jour. Geol. Soc, xxvii. (1871) p. 296. 



