152 The Work of the Geological Survey. 
formation as to the composition of the mineral masses encountered 
in the field. 
When an officer engaged in mapping meets with rocks which 
present difficulties, either as to their classification or as to their 
bearings on the structure of the ground, he takes specimens of them, 
which he numbers consecutively and sends up to the petrographer 
at the office, who enters them in a book under the name of that officer, 
and keeps a record of the destination of each. Those specimens 
which are selected to be sliced are numbered consecutively in the 
order in which they are cut, and are entered in books kept for the 
purpose. When they have been microscopically studied, described, 
and named, they are again entered in two distinct catalogues, one 
of which is arranged according to the sheets of the 1-inch map, 
and the other according to petrographical types. Every sliced 
specimen is thus entered four times, and every specimen sent up for 
examination (whether sliced or not) can at once be found. A report 
is made out by the petrographer and sent back to the officer, who is 
thus put in possession of all the details which can be furnished to 
him regarding the rocks about which he needed assistance. In 
many cases the thin slices are also sent to the surveyor, who often 
spends his evenings in the study. 
The original specimens from which the thin slides have been 
prepared are carefully kept in cabinets, so that if any accident 
should befall a slide a new slice can at once be cut. The mounted 
slides are arranged in separate cabinets. A large number of such 
slides have now been accumulated. From Scotland alone upwards 
of 5,000 have been determined, and are ready for reference at any 
moment. 
But besides assisting the field-work, the petrographers are 
engaged in determinations required for the arrangement of rock- 
specimens in the museums at Jermyn Street, at Edinburgh, and in 
Dublin. The collectors are employed under the supervision of the 
surveying officers to make illustrative series of specimens of the rocks 
of each district. These are sent up to the office for examination 
and for insertion in the museums. In the course of the research 
thus imposed on them the petrographers are from time to time 
enabled to make important original contributions to petrographical 
science. Moreover, they confer in the field with the officers who are 
engaged in mapping, and sometimes in concert with them make 
observations which are embodied in conjoint Memoirs on the geology 
of the districts. 
III. Palaeontological Work. 
In a country where the geological formations are to a large 
extent fossiliferous, it is necessary to pay close attention to the 
organic remains found in the rocks, to collect specimens of them, to 
determine these specifically, and to regulate thereby the geological 
boundary-lines upon the maps. The duty of examining and 
reporting upon the fossils is entrusted to the palaeontologists, who 
