PROGRESS OF MICROSCOPICAL SCIENCE. 183 



that he is justified in maintaining the old description, because the 

 microscopes emi^loyed by " naturalists " do not exhibit the peculiar 

 si)heres of silica ! ! This is, it seems to us, a mistake, but of course 

 it is a trivial one. The synonyms ajjpear to be fairly stated; but 

 we fancy that the distribution might in some cases be a little more 

 fully given. On the whole, we may fairly say that the work is well 

 done, and we may hoi^c that the futm-o parts will, if anything, exceed 

 in value the jiresent one. 



PEOGEESS OF MICKOSCOPICAL SCIENCE. 



The Microscopic Characters of certain Mochs. — Geologists are gradu- 

 ally becoming more accustomed to the microscope in the practice of 

 their profession. It is becoming essential for every geologist to use 

 this instrument when possible in the examination of rocks. We have 

 before us a reprint from the ' Philosophical Magazine ' (February) of a 

 paper by Mr. J. Arthur Phillips, F.C.S., on the microscopic character 

 of certain Cornish rocks, which aifords ample testimony of the value of 

 the instrument. The microscopical specimens on which the author 

 si)eaks have been prepared in the usual way, but the softer varieties of 

 killas have received exceedingly careful manipulation. In the case of 

 the more fissile specimens, it was often found very difficult to obtain 

 satisfactory sections at right angles to the cleavage-planes ; and even 

 when procured, great care was required in mounting, to prevent their 

 entire disintegration in the balsam employed for that purpose. Each 

 section was examined by the aid of two different arrangements of the 

 instrument. The first, spoken of as " a low power," magnified 60, and 

 the second, mentioned as " a high power," about 400 linear. The 

 polariscope could be conveniently used with both. Higher powers 

 were sometimes, but not often, employed. The author says that thin 

 sections of the " killas from Polgooth Mine," when examined by 

 transmitted light under a low power, exhibit no decided evidence of 

 structure, but apj)ear as a milky- white mass in which are disseminated 

 numerous moss-like semi-crystalline markings of a brownish-green 

 colour. They are also observed to be traversed by various fissui-es, 

 which have become filled by crystalline transparent quartz. When 

 examined by a higher power this rock is found to consist of an aggre- 

 gate of minute granules intimately blended together, and without 

 definite outlines. There were also some grains of oxide of iron and 

 other dark markings not sufficiently opaque for this substance, some 

 are apparently hornblende, and others are probably jjatches of a 

 chloride mineral. This slate has undergone so great a change that 

 the original grains of which it is composed can only be made out with 

 the polariscope. Sjoeaking of killas from Polmear Mine, 40 fathoms 

 below the surface, the author having- first given the chemical composi- 

 tion, says that sections prepared from this rock do not differ materi- 

 ally from those obtained from the Polgooth specimens. The ultimate 



