Yol. 50.] PEELITIC AND SPnERULITIC STRCCTOru-S. 



13 



tanoously developed, but I am unable to quote any example in 

 which there is proof of this. 1 The man who can first develop 

 perlitic structure in a crj'stalline mass will do much to upset what 

 has hitherto been written concerning vitreous rocks. It is to be 

 hoped that experiments in this direction may be tried by those who 

 anticipate successful results. 



It is possible that time may show that the views entertained by 

 Messrs. Harker and Marr are correct upon this point, but, in the 

 absence of further evidence, I adhere to my original statements. 



EXPLANATION OF PLATE I. 



Perlitic and Sphenditic Structures in Vitreous and Devitrified Lavas. 



Order of 



development 



of structures. 



1st, 



Perlitic. 



2nd, 



Spherulitic, 



also in places 



Microcrystal- 



line. 



1st, 

 Spherulitic. 



2nd, 



Perlitic or 



Synchronous. 



1st, 



Perlitic. 



2nd, 



Spherulitic. 



1st, 



Spherulitic. 



2nd, 



Perlitic. 



3rd, 



Microerystal- 



line. 



Fig. 1. Long Sleddale, Westmorelnnd. Perlitic cracks, x 140. 



Fig. 2. The same cracks as those shown in fig. 1, but with 

 the positions of three spherulites, as seen between 

 crossed nicols. It is noteworthy that, where these 

 spherulites have been formed across the perlitic 

 fissures, the latter are often barely perceptible, or 

 are completely obliterated. For the sake of clear- 

 ness the other spherulites and microcrystalline 

 grains, by which the rock is totally devitrified, have 

 been omitted. Had the entire field been represented 

 as filled with spherulites, the point emphasized in 

 the text would not have been apparent. 



Fig. 3. Yellowstone, Montana, U.S.A. Spherulites surrounded 

 by perlitic cracks. These spherulites give a well- 

 defined dark cross in polarized light, x 140. 



Fig. 4. Buschbad, near Meissen. Perlitic cracks traversing 

 spherulites (s). The latter are of later formation 

 than the cracks. X 30. 



Fig. 5. Boulay Bay, Jersey. On the right-hand, perlitic cracks 

 in devitrified obsidian (microcrystalline) : on the 

 left, part of a large spherulite (s) which is not tra- 

 versed by the perlitic structure. X 30. 



Discussion. 



Mr. Marr thanked the Author for the very courteous manner 

 in which he had spoken of his opponents. He was pleased to 

 have elicited this interesting communication from Mr. Rutley, but 

 regretted the absence of his colleague Mr. Harker, which neces- 

 sitated his making an attempt to reply to the paper. With regard 

 to the Long Sleddale rock, he thought it was simpler to understand 

 perlitic cracks being arrested by spherulites than to suppose that 

 the spherulites formed across and obliterated pre-existing perlitic 

 cracks. The confident way in which the Author had spoken of the 

 Meissen rock seemed to imply that he was, even now, not so confident 



1 The obsidian from the Yellowstone (PI 

 both structures simultaneously developed. 



I. fig. 3) may possibly have had 



