Vol.50.] PERLITIC AND SPIIERULITIC STRUCTURES. H 



to pass unchallenged ; but, on casually looking through the pages of 

 the first part of the new edition of Prof. Zirkel's ' Lehrbuch der 

 Petrographie ' and finding therein an allusion to Messrs. Harker 

 and Marr's criticism of my views, it seemed incumbent upon me to 

 defend my statements. 



Owing to the length of their very admirable paper, it is possible 

 that the particular passage in question was not read at the meeting. 

 Had it been, I should probably have responded during the discussion. 

 The point at issue is, in a certain sense, a small one, since the 

 structures themselves are usually microscopic ; but its significance is 

 larger than might, at first sight, appear, since it involves the re- 

 tention or abolition of an old-established landmark in petrography. 



In approaching this particular question we have to consider what 

 a rock once was, as well as what it now is. And, here, a difficulty 

 to which I have often alluded steps in. 



We have to distinguish between those rocks in which a micro- 

 or cryptocrystalline structure has been set up prior to, or during 

 consolidation, as in lithoidal rhyolites ; and those in which such 

 structure has been developed after consolidation, as in devi trifled 

 glassy rhyolites and obsidians. 



Among the older rhyolitic rocks there is but one structure, the 

 perlitic, which, when present, affords what has hitherto been re- 

 garded as a certain proof that a rock assumed a vitreous character 

 at the time of consolidation. It is a valuable means of diagnosis, 

 which must hold good until it can be pi'oved that perlitic structure 

 can be set up in a rock which already possesses a micro- or a crypto- 

 crystalline structure. 



If we examine the more recent rhyolites, and compare those of 

 a vitreous with those of a lithoidal character, we find that the 

 former frequently exhibit a perlitic structure, while in the latter no 

 such structure is ever seen, assuming, of course, that the lithoidal 

 character is not the result of subsequent alteration. 



I am unacquainted with a single instance in which this structure 

 has been developed in a recent lithoidal rhyolite, even when the 

 rock is mainly micro- or cryptocrystalline, still less would one expect 

 to find it in a rock essentially composed of small spherulites. 



In a section of an obsidian from the Yellowstone, largely com- 

 posed of minute spherulites traversing the rock in bands, a perlitic 

 structure has been set up in the vitreous portions of the rock, but 

 that it has been developed subsequently to the formation of the 

 spherulites is sufficiently proved by the way in which the perlitic 

 cracks here and there encircle an isolated spherulite and by the 

 manner in which the larger cracks follow the boundaries of the 

 spherulitic bands (PI. I. fig. 3). In another rock from the Upper 

 Geyser Basin, Madison River, Yellowstone, which is almost wholly 

 composed of small spherulites, there is no evidence whatever of 

 perlitic structure. In another similar rock from the Lower Geyser 

 Basin, perlitic structure is also absent. These are beautifully fresh 

 examples, aud should show the structure well if it were present. In 



