Geological Society of London. 425 



tion of an ancient crater by means of the intersection or concurrence 

 of the lines of apparent " orientation " of dykes alone. The author 

 stated that the intrusion of volcanic dykes cannot so greatly in- 

 fluence the slope of volcanic mountains as has been supposed. 



13. "On the Metamorphic Eocks surrounding the Land's End 

 Mass of Granite." By S. Allport, Esq., F.G.S. 



In this paper the author described the results of a microscopic ex- 

 amination of certain metamorphic rocks surrounding the Land's End 

 granite, indicating the changes produced by the intrusion of the 

 latter upon clay-slate and upon certain igneous rocks. The slates 

 in contact with granite become converted into tourmaline- and mica- 

 schists, and are found to contain crystalline quartz, tourmaline, and 

 three distinct varieties of mica, with occasionally tremolite, magne- 

 tite (and andalusite ?), and in some localities felspar. Their struc- 

 ture is also changed, the most remarkable changes being foliation 

 with every gradation from nearly straight parallel lines to the most 

 complicated contortions, and concretionary structure by segregation 

 of quartz and mica, the result being a spotted schist. The strata 

 near the granite contain far m.ore quartz than those at a distance ; 

 and the author thought that there, could be no doubt that much of 

 the quartz has been derived directly from the intruded rock. He 

 referred particularly to the fluid-cavities contained in the quartz of 

 the granite, schorl-rock and altered slates ; and from his observa- 

 tions upon them, stated that he was compelled to dissent from the 

 views of Mr. Sorby, inasmuch as he found no uniformity to prevail 

 in the relative sizes of the bubbles and fluid-cavities in the quartz 

 crystals belonging to precisely the same portion of rock, or even in 

 the same crystals. Hence he regarded it as impossible to arrive at 

 even an approximate estimate of either the temperature or the 

 pressure under which a given rock was formed from a consideration 

 of such characters. The author next described the characters and 

 mode of occurrence of tourmaline both in the granite and in the 

 schorl-rock, and inferred that at the separation of the latter from 

 the former the whole mass was in a plastic state, and that then the 

 tourmaline and quartz became crystallized in an order varying in 

 accordance with varying conditions. He also noticed the alteration 

 of tourmaline producing pseudomorphs of that mineral ; and stated 

 that while all the three varieties of mica found distinctly furnish 

 the red lithium-line when treated spectroscopically, this is most 

 strongly marked and persistent in the white variety, which is prob- 

 ably a typical lepidolite. 



The altered dolerites and basalts described by the author in the 

 remainder of his paper are marked as '' Greenstones " on the map of 

 the Geological Survey. They were stated to vary in colour from 

 dark bluish green to dark brownish green, and in texture from 

 coarsely crystalline rocks, not fissile in any direction, to fine-grained 

 or compact rocks with an imperfect slaty cleavage. The coarsely 

 crystalline rocks are regarded by the author as altered dolerites, and 

 some, if not all, of the more compact varieties, as originating from 

 fine-grained basaltic portions of the same rocks. The author was 



