Mr. Allport on the Rocks of the LanoV s-End Granite. 469 



13. "On the Metamorphic Rocks surrounding the Land's -End 

 mass of Granite." By S. Allport, Esq., E.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 more 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 gra- 

 nite, schorl-rock, and altered slates, and, from his observations 

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

 of llr. 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 ap- 

 proximate estimate of either the temperature or the pressure under 

 which a given rock was formed from a consideration of such charac- 

 ters. The author next described the characters and mode of occur- 

 rence 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 pro- 

 ducing 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 per- 

 sistent in the white variety, which is probably a typical lepidolite. 



The altered dole rites 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 

 unable to decide whether these masses are contemporaneous andin- 

 terbedded, or intrusive sheets (both may be present) ; but in any 

 case they are older than the granite. He described in some detail the 

 structure of specimens of these altered rocks from many localities, 



