THE LOWER-SILURIAN DISTRICT OF SHROPSHIRE. 451 



of the mass, the rock appears to present the same general characters 

 throughout, specimens from the northern end being quite similar to 

 some of those collected at the opposite extremity. It is for the most 

 part a hard compact rock of dark red or brown colour, and is the 

 compact felspar and homstone of Murchison, who also refers to one 

 variety as a porphyritic clinkstone. 



In order to show clearly the true character and structure of these 

 rocks, it will be necessary, in the first place, to give a short account 

 of their recent analogues ; as rocks of this peculiar type are by no 

 means of wide distribution, and some of them have not been previ- 

 ously observed in these islands. 



II. Structure of Modern Perlites. 



Perlite, spherulitic perlite, perlitic pitchstone, and perlitic obsidian 

 belong, as is well known, to the glassy group of acid rocks, their 

 average amount of silica being at least 70 per cent. In typical un- 

 altered specimens the mass consists of a true glass which has no 

 action on polarized light. 



The spheroidal and ellipsoidal balls by which perlite is chiefly cha- 

 racterized have been described by Zirkel *, Rosenbusch f , and 

 Lassaulx % as consisting of concentric laminae arranged like the coats 

 of an onion — a comparison which may, I think, possibly lead to 

 erroneous ideas as to their real character ; for however close may be 

 the resemblance, as seen in thin sections, there is no real analogy 

 between the structure of these perlitic spheroids and that of a tuni- 

 cated bulb built up of broad scales which surround each other in a 

 concentric manner. 



An examination of typical specimens from the old volcanic districts 

 of Schemnitz in Hungary, Meissen in Saxony, and Cabo de Gata on 

 the south coast of Spain shows that the spheroids have not been 

 formed by the superposition of successive laminae : they are not con- 

 cretions in any sense of the term ; nor is there any thing about them 

 suggestive of any process of progressive construction. When a rather 

 thick section is examined under a low power of the microscope, it is 

 at once seen that the little spheroidal balls are merely portions of the 

 homogeneous glass which have been partially separated from the 

 general mass by the formation of a number of small curved planes 

 of fracture ; these are more or less concentric with each other, but 

 vary greatly in size, and are irregularly disposed in various directions 

 round the centre. As these curved planes lie at various depths in 

 the section, some of them appear with a convex or concave surface, 

 according as the slice happens to cut through the upper or lower half 

 of a spheroid. Such being the general arrangement, the lines seldom 

 form closed curves when seen in thin sections. That these lines and 

 curved planes are really fine cracks is clearly shown by the way in 

 which they are frequently filled more or less completely by the in- 



* Mikroscop. Beschaff. dei* Mineralien und Gesteine, p. 365. 

 t Miki'oscop. Physiographie, p. 124. 

 \ Elemente der Petrograpkie, p. 221. 



