hy : ap CF a Pe 
é 3 ae Pa yay iy ; Fok tae 
ea ‘ < +z > . mt . ~~ eX 
De : iS? 3a 7 at fay 
ey aa GEOGNOSY. - [Boox II. | 
canic tuffs and breccias are characterized by the occurrence of lapilli 
(very commonly cellular) of the lavas from the explosion of which 
they have been formed (p. 163). Among interbedded volcanic rocks the 
student will meet with beds which he may be at a loss whether to class 
as volcanic or as formed of ordinary sediment. ‘They consist of an inter- 
mixture of volcanic detritus with sand or mud, and pass on the one side 
into true tuffs, on the other into sandstones, shales, limestones, &c. If the 
component fragments of a non-crystalline rock give a brisk effervescence — 
with acid they are calcareous, and the rock (most likely a limestone, or 
at least a calcareous formation,) should be searched for traces of fossils. 
2. The Paste.-—It sometimes happens that the component fragments 
of a clastic rock cohere merely from pressure and without any discover- 
able matrix. This is occasionally the case with sandstone. Most com- 
monly, however, there is some cementing paste. If a drop of weak acid 
produces effervescence from between the component non-calcareous 
grains of a rock, the paste is calcareous. If the grains are coated with 
a red crust which on being bruised between white paper gives a cherry- 
red powder, the cementing material is the anhydrous peroxide of iron. 
If the paste is yellow or brown, it is probably in great part the hydrous 
peroxide of iron. A dark brown or black matrix which can be dissipated 
by heating is bituminous. Where the component grains are so firmly 
cemented in an exceedingly hard matrix that they break across rather — 
than separate from each other when the stone is fractured, the paste is 
probably siliceous. 

ii. Microscopic Investigation.! 
The value of the microscope as an aid in geological research has — 
been sufficiently dwelt upon in the preceding pages. Some informa- 
tion may now be given as to the methods of procedure in microscopical — 
inquiry. 
: i; fee dection of microscopic slides of rocks and minerals. 
—The observer ought to be able to prepare his own slices, and in 
many cases will find it of advantage to do so, or at least personally: 
to superintend their preparation by others. It is desirable that he 
should know at the outset that no costly or unwieldy set of apparatus 
is needful for his purpose. If he is resident in one place and can ac- 
commodate a cutting machine, such as a lapidary’s lathe, he will 
find the process of preparing rock-slices greatly facilitated.27 The 
* This section is taken, with alterations and additions, from the author’s Outlines of 
Field Geology. 
* A machine well adapted for both cutting and polishing was devised some years _ 
azo by Mr. J. B. Jordan, and may be had of Messrs. Cotton and Johnson, Grafton 
Street, Soho, London, for £10 10s. Another slicing and polishing machine, invented 
by Mr. F. G. Cuttell, 52 New Compton Strect, Soho, London, costs £6 10s. These 
machines are too unwieldy to be carried about the country by a field-geologist. Fuess 
of Berlin supplies two small and convenient hand-instruments, one for slicing, the other 
for grinding and polishing. ‘The slicing-machine is not quite so satisfactory for hard 
rocks as one of the larger more solid forms of apparatus worked by a treadle. But the 
grinding-machine is useful, and might be added to a geologist’s outfit without material 
inconvenience. If a lapidary is within reach, much of the more irksome part of the work 
may be saved by getting him to cut off the thin slices in directions marked for him 
upon the specimens. 

