29 



impossible suppositions, or else to be quite vague and unsatis- 

 factory. 



As for the spheroidal concretionary theory, he believes it to be 

 founded on a total mistake. He regards the spheroids, so often 

 met with in decaying basalts or lavas, as being not concretions at 

 all, but as being the results of decay or decomposition penetrating 

 from without inwards in blocks into which the rock has been 

 divided by fissures, which may have arisen from various causes. 

 From this, and other reasons, which in his lecture he stated at 

 length, he is led to give no credence whatever to the spheroidal 

 concretionary theory of the jointed prismatic structure.* 



* To describe adequately and to discuss the views of the principal geologists 

 on the jointed prismatic structure and its origin, would greatly exceed the limits 

 to which the present abstract must be confined. The annexed references at the 

 end of this note to passages in the writings of several geologists will, however, 

 serve sufficiently to guide to all the principal views on the subject, as expressed by 

 writers who themselves have proposed or maintained them : or have, at least, 

 promulgated them with favour, though in some cases with vagueness — the 

 natural result, as it appears to Professor Thomson, of attempting to explain 

 and propound intrinsically untenable views. Sir Charles Lyell's writing, for 

 instance, in his Elements of Geology, under the heading Columnar and Globular 

 Structure, while appearing to put forward with favour the spheroidal concre- 

 tionary theory, appears also to be affected with this character of vagueness. 

 Dr. Daubeny, Beete Jukes, and several others, with much more boldness main- 

 tain the spheroidal concretionary theory, under various modifications of its 

 details ; while Mr. Scrope repudiates it, denying that the columnar structure 

 could have had its origin in spheroids pressing against each other, and main- 

 taining that the columns have originated through "Assuring" of the hot rock, 

 by ^'contraction" during its process of "consolidation" or^" refrigeration;" and 

 accounting for the cross-joints in a way which will be alluded to in a following 

 note in this paper, and which the writer, Professor Thomson, considers to be 

 without doubt totally untenable. 



REFERENCES. 

 Daubeny, On Volcanoes. 2nd Edition, 1848. pp., 65, 78, 79, 660, 661, and 680. 

 Scrope, on Volcanoes. 2nd Edition, 1862. pp., 93 to 106. 

 Lyell, Elements of Geology. 6th Edition. 1865. pp., 610 to 613. 

 Jukes, Popular Physical Geology. 1853. pp., 109 to 113. 

 Page, Advanced Text Book of Geology. 2nd Edition, 1859. p., 72. 

 Du La Beche, Geological Observer. 2nd Edition, 1853. PP-» 4°4 t0 4°7- 

 Dana, Manual of Geology. Revised Edition,, 1865. pp., 97 to 99, 626 and 627. 

 Carl Vogt. Lerbuch der Geologic 1854. VoL II., p. 223. 



