THE NATURAL HISTORY OF IGNEOUS ROCKS. 13 



most fundamental points concerning them are still involved 

 in doubt. Their geographical and local distribution is one 

 such question. It is a familiar fact that the arrangement of 

 volcanic vents at the present day is in general a linear one, 

 giving place in some regions to a disposition in groups ; 

 further, that the lines often correspond very evidently with 

 coast-lines, while the groups are often island-groups. On 

 these and other data some have based theories oi' the situa- 

 tion of the vents on lines of fracture in the earth's crust, 

 while others have speculated upon an accession of sea- water 

 as the proximate cause of volcanic eruptions ; but such 

 hypotheses rest at present upon a very slender basis of 

 ascertained fact. Other questions arise when we examine 

 the products ejected by modern volcanoes. On the one 

 hand, we find throughout a vast tract, such as the Andes, 

 all the volcanic rocks closely related in characters and de- 

 parting but little from a central type ; on the other hand, a 

 small group, such as that of the yEolian Isles, may afford 

 varieties of lava of widely diverse and highly special 

 characters. To such problems a study of modern volcanoes 

 alone can scarcely hope to bring a satisfactory solution. It 

 is necessary to inquire whether some empirical laws which 

 seem to hold good are not rather to be regarded as parts of 

 larger principles, and to seek for an explanation of present 

 anomalies by viewing them as survivals of a pre-existing 

 state of things. 



In such a course, as in not a few modern geological 

 inquiries, we are reversing the maxim of the school of 

 Hutton and Lyell. Instead of applying a knowledge of 

 the processes now going on around us to elucidate the 

 record of past ages, we are making use of the history of 

 the past to explain the phenomena of the present. Geology 

 is thus repaying some of the debt which she owes to Physical 

 Geography. 



The frequent close connection of plutonic and intrusive 

 rocks with surface vulcanicity is now scarcely called in 

 question, though, in the nature of the case, instances must 

 be few in which such connection can be directly demon- 

 strated. Special stress was laid on this point by Judd in 



