[26 



NATURE 



{A 21 gust 5, i88o 



appears to be followed without interruption by a con- 

 formable suite of Eocene strata. 



The geographical changes that closed the Cretaceous 

 period in the West were among the most important in 

 the evolution of the American continent. Over many 

 thousand square miles the floor of the sea was raised 

 into land which has never since been again submerged. 

 The lacustrine conditions which began in Cretaceous 

 times now received a far greater development. The 

 waters of the ocean, inclosed into inland seas, from 

 brackish became fresh, and one or more lakes, of per- 

 haps even greater dimensions than those of Eastern 

 America, stretched between the heights of the Great 

 Basin and the Rocky Mountains for as yet an unknown 

 distance to the south. The history of these lakes has 

 been studied by Hayden, King, Powell, and other geolo- 

 gists, and their marvellously rich ichthyic, reptilian, and 

 mammalian fauna has been described by Leidy, Marsh, 

 and Cope. Much remains to be done before the history 

 can be regarded as even approximately filled in. In the 

 meanwhile it is certain that this lacustrine area was 

 undergoing slow subsidence during Eocene time, that 

 sediment was being continually washed into it from ad- 

 joining mountains, that eventually 5,ooo feet or more of 

 strata were laid down over its site, and that the area of 

 fresh water progressively diminished. 



A new chapter in this eventful history is revealed by 

 Capt. Dutton. He tells how in Southern Utah the 

 lake, even as far back as the time of the Middle Eocene, 

 was the theatre of volcanic discharges, and how these, 

 after vast intervals of cjuiescence and almost incredible 

 denudation, have been from time to time renewed down 

 even to a period so recent that it can hardly be believed 

 to date 50 far back as the days of Cortez and the Spanish 

 Conquest. He shows that this volcanic district discloses 

 a remarkable variety of phenomena, nearly every form of 

 eruption being exhibited, and every great group of vol- 

 canic rocks being represented in it. The earliest volcanic 

 rocks are tuffs, which he regards as probably derived in 

 chief measure from the degradation of older lavas and 

 the deposit of the resulting sediment on the floor of the 

 lake. The next phase of volcanic activity was marked 

 by the outpouring of masses of propylite and hornblende- 

 andesite, and was succeeded by the third and grandest of 

 all, when floods of trachytes and rhyolites, alternating 

 with augitic andesites and dolerites, rolled far and wide 

 over the plateaux. The author is doubtful whether these 

 extravasations proceeded from /Etna-like summits or 

 craters, and is rather inclined to look upon the larger 

 deluges as having issued from local fissures. Certainly 

 if any true lofty volcanic cones existed, all external trace 

 of them has been completely effaced by denudation. 

 The closing event in this long volcanic period, if indeed 

 the record can be properly regarded as even yet closed, 

 consisted in the emission of abundant streams of lava 

 round the larger areas of previous activity. Capt. Dutton 

 notices some remarkable examples of a feature which 

 occurs on a much smaller scale in the volcanic region of 

 the Rhine and Moselle. The basalt cones and craters 

 whence the streams have emanated seldom appear at the 

 base of the great cliffs or at the bottoms of the deep 

 canons. They are often crowded together near the 

 crests of the terrace walls, or the lava has broken out 

 from the face of a wall. They commonly lie near lines 

 of fault, yet appear almost always on the uplifted instead 

 of the depressed side of the dislocation. "The least 

 common place for a basaltic crater is at the base of a 

 cliff." Among the volcanic masses special attention is 

 given to the enormous accumulations of conglomerate 

 and tuff, which cover nearly 2,000 square miles of area, 

 and range from a few hundred feet to nearly 2, 500 feet in 

 thickness. These vast piles of coarse detritus the author 

 attributes to the atmospheric disintegration of previously 

 erupted lavas, and he describes in detail the process by 



which similar conglomerates are at the present moment 

 being formed by frost, rain, and mountain-torrents. The 

 highly important observation was made by him among 

 the older tuffs, that in some places they have been so 

 metamorphosed that the product of alteration is a rock 

 possessing all the ordinary characters of a lava. 



The chronological sequence of volcanic rocks among 

 the Plateaux of Utah has been recognised as obeying 

 generally the order enunciated by Richthofen. Capf. 

 Dutton, starting from this observed sequence, devotes 

 two long chapters to theoretical discussion — one on the 

 classification, the other on the origin of volcanic rocks. 

 To his work in the field he has added careful labour 

 indoors, especially studying the microscopical and chemical 

 characters of volcanic rocks. No one can read his 

 pages without recognising their suggestiveness, even 

 though the conclusions reached in them may sometimes 

 appear doubtfully valid. His remarks upon the texture 

 of volcanic rocks (pp. 91-99) offer an excellent sample of 

 his critical treatment. Pointing out how different rnay 

 be the texture assumed by the same original magma 

 according to whether the mass has cooled and consoli- 

 dated at the surface or beneath it. he is disposed to regard 

 the intrusive condition as a kind of intermediate stage 

 between volcanic rocks which have issued above ground 

 and non-eruptive masses which have remained inactive 

 deep beneath it, and he regards the porphyritic texture as 

 especially characteristic of this " qualified eruption." This 

 generalisation is only partially supported by the volcanic 

 history of Britain. Among our older Palaeozoic rocks, 

 indeed, the intrusive or injected masses very generally 

 possess the porphyritic structure. But from the time of 

 the Lower Old Red Sandstone onwards to the Miocene 

 volcanic period inclusive, the intrusive sheets are for the 

 most part non-porphyritic, w-hile the porphyritic structure 

 is found among the superficial lavas. The classification 

 our author proposes is as follows : — 

 ACID SERIES— Group I. Rhyolites. 



Sub-group I. Nevadite or granitoid rhyolite. 



2. Liparite or porphyritic rhyolite. 



3. Rhyolite proper, or hyaline rhyolite. 



SUB-ACID SERIES— Group II. Tr.\chytes. 

 Sub-gronp A. Sanidine Trachytes. 



1. Granitoid Trachyte. 



2. Porphyritic Trachyte. 



3. ArgiUoid Trachyte. 



4. Hyaline Trachyte. 

 Sub-group B. Hornblendic Trachytes. 



5. Hornblendic Trachyte. 



6. Augitic Trachyte. 



7. Phonolite. 



8. Trachytic Obsidian. 



SUE-BASIC SERIES— Propylite and Andesite. 



Sub-group I. Hornblendic Propylite. 

 .2. Augitic Propylite (?). 



3. Quartz-Propylite. 



4. Hornblendic Andesite. 



5. Augitic Andesite. 



6. Dacite or Quartz-andesite. 



BASIC SERIE.S— B.\salts. 



Sub-group I. Dolerite. 



2. Nepheline-dolerite. 



3. Basalt. 



4. Leucite-basalt. 



5. Nepheline-basalt. 



6. Tachylite. 



The fifth chapter is entitled "Speculations concerning 

 the Causes of Volcanic .^Xction." The author propounds 

 a very ingenious trial hypothesis, by which he believes 

 the sequence of volcanic phenomena throughout at least 

 the Rocky Mountain region may be explained. He 

 assumes that volcanic phenomena are brought about by a 



