Becker — Volcanic Cones and Elastic Limit of Lava. 283 



"We may regard the primary division into Laurentian and 

 Huronian as thoroughly established, and hence are warranted 

 in using lithological distinctions to guide us in discriminating 

 the age of crystalline rocks so far as they can be derived from 

 the predominant features of these great systems. For a long 

 time to come their further subdivisions will be open to question. 



Art. XXX VII. — The Geometrical Form of Volcanic Cones and 

 the Elastic Limit of Lava ; by George F. Becker. 



General character of a volcanic cone. — If a fluid or partially 

 fluid substance, such as lava, issues from an orifice in a plain 

 and congeals about the mouth, the accumulation will form an 

 elevated mass more or less nearly resembling a cone. If the 

 resistance of this solid at the base is greater than the load, 

 matter will or may continue to accumulate on the upper por- 

 tion, while if the resistance towards the base is smaller than 

 the load, a readjustment of material will necessarily take place. 

 It is to be inferred, therefore, that under favorable circumstances 

 volcanic cones will approximate to some definite form, and this 

 inference is strengthened by the well known fact that the form 

 of craters has been closely imitated by experiments on plastic 

 material. Observation, too, shows that volcanic cones are char- 

 acterized by a remarkable regularity of outline and that the 

 outlines of the more regular cones of volcanoes wherever ob- 

 served are strikingly similar. 



Milne's results. — Processor John Milne has discussed the 

 form of volcanic cones,* but his papers have not received the 

 attention they deserve, containing, so far as I know, the first 

 attempt to assign a definite geometrical form to a natural 

 surface, f Though his papers are cited in some recent works 

 on geology, I have seen no reference to this fact and it had 

 entirely escaped my own attention until the material for a 

 paper on the subject was completed. My results are closely 



* Geol. Mag., vol. v, 1878, p. 338; and vol. vi, 1879, p. 506. 



f Geology is one of the least exact of sciences, and opinions still differ among 

 the ablest specialists as to its most fundamental doctrines ; but, so far as geo- 

 logical relations are exactly known, they are necessarily capable of mathematical 

 expression, and all relations are essentially exact, however imperfectly we may 

 be acquainted with them. Per contra, the investigations of Sir William Thomson, 

 Professor G. H. Darwin and other physicists show what grand additions may be 

 made to geological science by the application of mathematical reasoning, and it 

 appears to me that geologists should be on the watch for every opportunity to 

 achieve for any geological relation, however insignificant it may appear, that 

 final intellectual conquest which is symbolized by its correct expression in mathe- 

 matical language. As in other branches of theoretical physics blunders will be 

 made and progress will be slow, but opinion will gradually yield to certainty over 

 a portion of the infinite field. 



