428 Reviews— H. F. Reid—The Miiir Glacier. 



latter the amphibole consolidated first, whereas in the amphibole- 

 pyroxene-andesite the amphibole and pyroxene crystallized more 

 or less simultaneously. 



Amphibole-andesite is rare. 



Among the Dacites the biotite-amphibole species is the most im- 

 portant. In this the felspar ranges between andesine and sanidine, 

 and is often altered to opal. The most interesting part of the 

 detailed description of this type is the discussion of the origin and 

 significance of the so-called " corrosion-figures " : he does not deny 

 that crystals are sometimes corroded by the still fluid magma, but 

 attributes many cases that would generally be attributed to this, to 

 other influences, such as original deformations, oscillation, twins, 

 mechanical strains, etc. A spherulitic structure, though of a some- 

 what imperfect type, occurs in this rock and in the pyroxene 

 andesite. In the pyroxene-amphibole andesites, andesine is the 

 chief plagioclase, while basaltic olivine is present as an accessory. 

 Pyroxene-dacite was only once met with. 



The last section of the " General Part " is devoted to the segrega- 

 tions, agglomerate-lavas, and ejected blocks and ashes. A felspar 

 basalt is represented by specimens from three localities. A chapter 

 is also devoted to the evidence afforded by 17 analyses, as to the 

 chemical relations of andesite and dacite ; the silicate percentage of 

 the former ranges from 54-21 to 6226 per cent. ; and that of the 

 latter from 63-36 to 70-22 per cent. 



The " Special Part " dealing with the geographical distribution of 

 rocks, occupies from pp. 89-192 ; the most interesting point brought 

 out in this is the great variation of the rocks at each volcanic centre : 

 thus andesites and dacites are associated, and usually in more than 

 one variety of each. 



The description of rock-specimens considered apart from their 

 field relations is never of course the ideal method of petrography, 

 as no doubt Dr. Kiich would be quite ready to admit ; but in the 

 case of such distant and comparatively inaccessible areas as the 

 Eepublic of Colombia, it is all that can be hoped for at present. 

 The detailed descriptions of the rocks, however, will greatly lighten 

 the labours of the lucky geologist to whose lot it may fall to work 

 out their relations in the field. We must accordingly be grateful to 

 Dr. Kiich for the care with which he has investigated the materials, 

 and to the two illustrious travellers whose careful collecting has 

 added so greatly to our knowledge of the geology of the northern 

 Andes. J. W. G. 



III. — The Mdir Glacier. 



" Studies of the Muir Glacier, Alaska. By Harry Fielding 

 Eeid. National Geographic Magazine, Vol. IV., pp. 19-84, 

 pis. i.-xvi. Washington, D. C, 1892. 



THE exploration of Alaska is being rapidly advanced by the 

 energy of the American geographers and geologists. The 

 latest addition is a detailed study of the Muir Glacier, made last 

 year by Professor Eeid, and which has resulted in the correction of 

 some very important errors in our previous knowledge of the 



