l888.] NEW-YORK MICROSCOPICAI, SOCIETY. 141 



demonstrated by the microscopical investigation of rocks border 

 on the marvellous. 



Minerals may be termed the individuals of the inorganic 

 world. Each has its characteristic features or properties, which 

 distinguish it from every other mineral. Quartz, feldspar and 

 mica, separately considered are minerals ; but, when occurring 

 in a state of mixture, they constitute a true rock — granite. And 

 it is the determination of these minerals, and of the conditions 

 under which they assumed the forms in which we now find 

 them, which is the chief aim of microscopical petrology. 



Thirty years ago, the only optical apparatus, employed by 

 geologists for the examination of rock-specimens, consisted of 

 an ordinary pocket-lens, with a magnifying power of from four 

 to fifteen diameters. This was deemed quite sufficient for all 

 ordinary requirements, and some even disdained the use of 

 magnifying glasses altogether. Specimens were only examined 

 externally. It was noted what kind of appearance a freshly 

 broken surface presented ; whether the specimens were rough 

 or smooth, coarse-grained or the reverse; what kind of odor they 

 emitted wlien breathed against; how they felt to the touch; 

 whether they yielded to the scratch of a piece of iron or the 

 finger-nail — in fact, the tests, which were deemed sufficient in 

 those days, appear quite ludicrous in the light of modern achieve- 

 ment. 



To Prof. Sorby, still living in England, is due, in a great ex- 

 measure, the credit of having first pointed out the fact that a 

 vast deal of information can be obtained from a microscopical 

 investigation of rock-specimens. Indeed, he may be called the 

 father of modern petrology. It occurred to him to prepare thin 

 slices of rocks, reducing them by grinding and other processes 

 to a state of extreme thinness, so as to render them more or less 

 transparent. These he mounted in Canada-balsam on glass- 

 slips, and placed them under his microscope, applying magnify- 

 ing powers of from 40 to 700 diameters. 



The result surpassed his most sanguine expectations. Dull 

 and shapeless stones, some picked up by the road-side, and 

 presenting to the naked eye nothing but a uniform tint of gray, 

 black or dirty-red, transformed themselves under the microscope 

 into fields strewn with beautiful crystals of wondrous colors and 

 forms, or literally blossoming as the rose. Every component 



