QUEKETT MICROSCOPICAL CLUB. 203 



mentary, and metamorpMc. The igneous rocks are found in the 

 greatest variety, and show the most interesting structures. Igneous 

 rocks were originally molten in the interior of the earth, and as 

 they cooled the various minerals crystallised out, generally those 

 with least silica crystallising first, and the others following, accord- 

 ing to the increase of silica. Gradual cooling, generally at a great 

 depth, produced coarse-grained rock (" plutonic "). If the 

 magma were injected into cracks in already consolidated rocks, it 

 cooled more quickly, and a finer-grained rock was the result. 

 These rocks are called " intrusive." If the mass were poured out 

 at the surface (" extrusive " or " volcanic " rocks), it cooled 

 very quickly, and sometimes solidified as glass without any crystals- 

 at all. From this it is seen that a molten magma having a definite 

 chemical composition may crystallise in three distinct forms,, 

 according to its environment. For instance, if it solidified at 

 great depths under heavy pressure, it would form a coarse-grained 

 granite ; intruded into fissures near the surface it might be a 

 quartz-porphyry, or pitchstone ; while if it were poured out at 

 the surface from a volcano, it would be a rhyolite. The nomen- 

 clature of rocks is based partly on the mineral content and partly 

 on the chemical composition. Igneous rocks are divided into four 

 groups, according to the silica content. Acid rocks contain over 

 66 per cent, of silica. Intermediate between 66 per cent, and 52 per 

 cent., Basic less than 52 per cent, and Ultrabasic less than 40 per 

 cent. It is difficult to settle the names, of the various types, as 

 they merge one into the other, so that the present names do not 

 represent definite chemical or mineralogical compounds. Mr. 

 Caffyn then showed on the screen a series of photographs illus- 

 trating the main types into which rocks are divided. Most of 

 the photographs were taken on Lumiere Autochrome plates by 

 Mr. Cafiyn and Mr. Ogilvy, and were most faithful representations 

 of the actual colours seen under the microscope. The magnifica- 

 tion in most cases was X 20 on the slide. The members much 

 appreciated the beautiful lantern slides, and a hearty vote of 

 thanks was accorded to Mr. Cafiyn for his interesting lecture. 



At the 552nd Ordinary Meeting of the Club, held on May 11th, 

 the President, Dr. A. B. Rendle, M.A., F.R.S., in the chair, the 

 minutes of the meeting held on April 13th were read and 

 confirmed. 



JouRN. Q. M. C, Series II.— No. 86. 14 



