Vol. 57.] OF THE TELLOWSTONE AND 6REA.T BRITAIN. 



221 



mica. These crystals seem to be strictly analogous to those found 

 projecting into cavities at Obsidian Cliff. 



Evidence with the microscope and in the field shows that here is a 

 series of rocks which were originally very vesicular lavas. A narrow 

 margin of spherulitic growth formed around the vesicles, which 

 appear either isolated or arranged by flow in a vermiform way 

 through the rock. Points of resemblance can be found in those 

 small blue spherulites of Obsidian Cliff which contain a central 

 cavity. It was mentioned in Pt. I (p. 212) that these have a layer 

 of brownish and somewhat friable material interposed between the 

 central hollow and the compact hard outer zone ; and probably it 

 will not be erroneous to conclude that the distinct zone surrounding 

 the vesicles of the Boulay-Bay rock is due to the presence of heated 

 vapour from which the adjacent rock-material was unable to free 

 itself during solidification. 



YII. Wrockwabdine (Shropshire). 



The rhyolites of the district near Wrockwardine are well known. 

 The greater part of the rock is of a dark chocolate-brown colour, as 

 are also the nodules and smaller spherulites. Frequently sur- 

 rounding the latter, and intermingled with the brown rock, is found 

 a dark-green, rather soft 



sjbsta nee, which is ap- Fig. 4. — Cavities of litJioph/sce from near 

 parently of the nature of Wrockwardine, now filled hy infiltrated 

 a residual glass. The silica : slightly larger than natural size. 

 nodules are usually lobate 

 in outline, and stand out 

 from a weathered surface 

 much less commonly than 

 is the case at Boulay Bay 

 and in Korth Wales, and 

 more frequently exhibit 

 lithophysal structure. 



This consists of a 

 system of rings analogous 

 to those from the Yellow- 

 stone region, which 

 closely follows the peri- 

 phery of the enclosing 

 nodule, and may or may 

 not surround a central 

 quartzose amygdaloid 

 <P1. YIIT, fi^.* 2 and 

 t^xt-fig. 4). 

 complicated 

 the entire 



-^ 



The very 

 form which 

 system fre- 



quently assumes is due, I believe, to pressure after solidification : 

 this pressure has broken down the concentric rings of rock, and 

 usually connected the cavities to the central hollow when such exists. 



