42 
Directions for Geological 
[Feb. 
Care must be taken to distinguish boulders or rolled masses from the out-crop- 
pings of the edges of the strata : the latter, being in their original position, are termed 
in situ ; whilst the former merely lie on the surface, and are often found at great 
distances from any rock of similar characters from which they may be supposed to 
have been derived. 
In examining the boulders, and also the pebbles of the beds of gravel, attention 
should be given to ascertain the correspondence or similarity of tlieir characters to 
the rocks of the neighbouring mountains. 
After having obtained a general idea of the rocks which prevail, specimens should 
be taken from the different beds or masses which are the most characteristic of the 
whole. It will be desirable that these specimens be kept to nearly the same size ; 3 
inches by 2 by 1 may be taken as average dimensions. 
In the beds of pebbles, a few of the different pebbles may be taken, and also spe- 
cimens of the beds of sand and clay ; but in these cases, description will be more 
requisite. 
Besides the specimens giving the general character of the whole, others should 
be taken of any particular varieties which occur ; as when the component simple 
minerals are larger and more distinct, or when minerals of a different appearance 
and character present themselves, either imbedded in the mass, or affixed on the 
edges of the cavities which happened to be in it. 
When such minerals are crystallized, it is an object to obtain all the different 
varieties of form, and particularly to select such as have the faces distinct and nu- 
merous, with the apex unbroken, and the crystal fixed in its original position in the 
rock, which is termed its matrix or gangue. In the cavities of the basaltic and amyg- 
daloidal rocks, crystallized minerals are of frequent occurrence : they are generally 
zeolites, or species belonging to the genus Kouphone Spar of Mobs. In collecting 
crystals, the minute ones are not to be neglected, for they have commonly their faces 
and edges the best defined and glancing ; and hence, from being capable of being 
measured by the reflective goniometer, are the most instructive to the mineralogist 
or crystallographer. Each specimen must be labelled with a reference to the lo- 
cality from which it has been taken •, for which purpose a rough tracing must be 
made from the plan or paper which will bear ink. On this tracing, for the geologi- 
cal plan, will be marked tlie points at which the strata come to the .surface, with 
their dip and bearing, changes of direction, intersection of veins or dykes, &c. 
The line of intersection of the plane of stratification with the plane of the horizon 
is the bearing, and the angle formed by the above two planes is the dip , which must 
therefore be measured in a plane perpendicular to the line of bearing, as it is evident 
that it is the direction of the greatest slope on which a ball or water would fall down 
the plane in free descent. 
In measuring the. above angle, it will be sufficient that it be done with a small 
wooden quadrant and plumb-line, or other simple contrivance, since the irregulari- 
ties of the strata do not admit of great nicety. The thickness of the beds should 
also be marked, but, for the same reason, measurement will not be required, provi- 
ed the dimensions be judged accurately whilst on the ground. When a number of 
these observations are collected, it is evident they will give the data, to make a 
geological section across an extent of country. 
A sect. on of the strata should also be made, when it can be done, on each of the 
geological plans to which the specimens are referred, and the same letters used, so 
that the label on the specimens may refer to both the plan and section. 
Ine portions where the strata or rock cannot be observed, should be left blank, 
as no supposition or uncertain observation or information is to be admitted. These 
tanks, however, may contain any remarks, either as to communication received, 
r inference made, with the reason assigned, 
oth J , sec /! on S0IU ? depth may be obtained when a shaft is sinking for a well, or 
°i “iV \ n ' v ^dch case a sketch of the beds, according to their order and thick- 
gi '' cn ’ wi,h f, 0 ,., e A 
Vallip« ^ i stratil ® re often exposed in the cliffs along the sea shore, or in 
-■» • — ’ " 1IC 1 Cases a sketch should be made, with specimens from the beds, their 
different • • 1 . . c,uu su oum oe made, with specimens noun me oeus, iue» 
The true •, ) ir° X |- llatt ' thicknesses marked, and their true inclination and bearing, 
in the sketri \ na '°i n ^ is evident, can scarcely ever be the same as that shown 
the iunrt; y Snt 1 ^duahons otten exhibit remarkable contortions in the strata, or 
the inlet-serf 0 10 r S -°* a < *^ rent k* n d» as basalt and limestone, sandstone, &c. or 
nied by sliS‘»nrf. vi!“ °\ dykeS throu 8 h the strata, (see fig. 1. 2. 3. PI. 1.) aceumpa- 
the beds in the mw 1 V • , le i° rmer signifying a depression or change of level of 
II m the of the dyke in a verticil direction ; the latter a sirni- 
