Henry Clifton Sorby. lix 
the potency of the microscope as an instrument of scientific research. His 
youthful studies in optics had, no doubt, made him familiar with all the 
details of its mechanism, and capable of adapting it to any line of investiga- 
tion in which he might find it of service. It was natural that he should 
first apply this instrument to’ the solution of some of the problems which 
his work among sedimentary rocks suggested. As far back as the year 
1831 a method had been devised and described by William Nicol of 
_ Edinburgh, whereby slices of fossil-wood could be mounted on glass and 
made so thin as to become transparent. In this form these sections were 
shown to reveal every detail of their internal organisation. Nearly twenty 
years passed before any geologist seems to have been induced to avail 
himself of this means of studying the minute structures of rocks. The 
first who did so was Sorby. He began by examining thin sections of the 
Caleareous Grit of the Yorkshire Coast, and he sent a communication on this 
subject to the Geological Society, which appeared in the ‘ Quarterly Journal ’ 
for 1851. As he advanced in his investigation of the microscopic characters 
of limestones and marls, he saw that the same method of study might be 
applied not only to sedimentary rocks, but to those of igneous origin, 
including even the most fine-grained and opaque. He thus entered a wholly 
new and untrodden field in geological enquiry. 
~The chemical composition of igneous rocks had long been investigated and 
was fairly well known. But though the component minerals of close- 
grained masses might be more or less probably surmised from the results of 
chemical analysis, no really accurate and precise information on this subject 
could thereby be obtained. In the early years of last century, indeed, 
Cordier had shown that by crushing fine-grained rocks and washing and 
separating the grains of their powder, their component. minerals might be 
isolated and examined under the microscope. But this method threw little 
or no light on the relations of these minerals to each other in the genesis of. 
the rocks composed of them. In 1858, however, Sorby’s great paper “ On 
the Microscopical Structure of Crystals indicating the origin of Minerals and 
Rocks” was published by the Geological Society. This masterly essay 
revealed that much of the origin and history of igneous rocks could be 
ascertained by means of the microscope. Not only was it shown to be now 
possible and easy to determine the several minerals, even in a close-grained 
rock, but to discover the order in which they had successively crystallised, 
the conditions of temperature and pressure in which their solidification had 
taken place, and the alterations which the rock composed of them had 
subsequently undergone. Sorby, besides making use of transmitted and 
reflected light, was familiar with the delicate applications of polarised light, 
and found with what great advantage they could be employed in the study 
of rocks. : 
- It was from this memorable paper that the remarkable modern develop- 
ment of the petrographical side of geology took its rise. In every country 
where the study of rocks is pursued, the methods first indicated by Sorby 
