APPENDIX. 869 



for 70°, 1.-3; for 60°, 1 : 2 ; for 50°, 1 : 1} ; for 45°, 1 : 1 ; f or 35°, 1] : 1 ; for 25°, 1J : 1 \ for 

 15°, 2: 1; and for horizontally, a crossed circle. 



If, among the exposed sections at a place, none is at right angles to the strike, the dip 

 may be obtained thus: take the dip and the direction along two of the sections; then, 

 from a point, A, draw two straight lines, A B, A C, in the directions of the observed 

 dips, and set off, on each, lengths proportional to the cotangent of its own dip; A b, A c ; 

 then, a line through b, c, will have the direction of the strike, and a perpendicular to it, 

 that of the dip. 



The best clinometer-compass has a diameter inside of 3 inches. The square base of 

 the instrument (a very important feature, as stated on page 95) has its sides parallel 

 with the N. S. and E. W. diameters of the compass, and is 3| inches on a side. Iu 

 taking the strike, a N. S. side of the base is made to coincide in direction with one of 

 the edges of the outcropping beds in case the exposed surface is horizontal, or with a 

 horizontal line drawn on a surface of one of the beds; but it is best to make the obser- 

 vation standing, by holding the instrument between the eye and the rock and obtain- 

 ing the coincidence by sighting across the N. S. side. The dip may be obtained, with. 

 such an instrument, from the under, as well as upper, surface of a projecting ledge, and 

 also from exposed sections scores of yards distant from the observer. (Instruments of 

 this kind are sold in New York by J. & H. J. Green, instrument makers). 



3. Unconformability. — Never confound the unconformity that is connected with a 

 fault with true unconformability, due to unconformable superposition. The different, 

 and differently-dipping, rocks on the opposite sides of a fault of a thousand feet or more 

 may belong to the same period. 



Never assert with positiveness that unconformability exists, unless the fact is dis- 

 tinctly visible in an actual section showing the contact of beds of unlike dip; for the 

 unlike dip in different rocks, if observed at points only a hundred feet apart, may be 

 owing to a bend in one or the other stratum in that interval, or to displacement. 



Observe the distinction between overlap (p. 101), and unconformability due to deposi- 

 tion on upturned strata. 



4. Metamorphic Rocks. — Study regions of metamorphic or granitoid rocks in pre- 

 cisely the same manner as those of ordinary stratified rocks, whether they be Archaean 

 or of later origin, using lithology in order to follow a series of rocks from mile to mile 

 over a country, but even this with caution, and always looking to stratigraphy and the 

 discovery of fossils for sure testimony as to relative age. Remember also, that a layer 

 of quartzyte may be gneiss or mica schist a few rods off; and that the same crystalline 

 rocks, with rare exceptions, may belong to formations of very various geological ages. 



As to granite, syenyte, and dioryte, make their age, in each case, a question to be 

 solved by careful stratigraphical investigation. In connection with the investigation 

 the following questions are to be answered: Is the rock eruptive granite, syenyte, or 

 dioryte ? Is the rock a vein formation ? Or, is the rock part of the metamorphic series 

 of a region, as proved by its mode of association with metamorphic rocks, and its gra- 

 dation into gneis-oid granite and gneiss, or into any other schistose crystalline rock? a 

 fact with much the larger part of the granite, syenyte, and dioryte of the world. 



5. Direction of Strike determined without an Instrument in hand. — It sometimes 

 happens that a geologist is in the field without his compass. His best resource in that 

 case, provided the sun is visible, is the following: Lay a leaf from a note-book over the 

 outcropping rock, with its surface quite level, and one edge (call it A B) in the direc- 

 tion of the strike. Sight the sun across it, or, better, cast the shadow of a plumb-line 

 over it (one made of a blade of grass weighted with a pebble might answer) and draw a 

 line corresponding with the sun's direction thus obtained. The angle between A B and 

 this line (which may be measured with a protractor) is the angle on the horizon between 

 the line of strike and the sun's meridian at the time. On turning, whenever convenient 

 afterward, to a book containing tables of the sun's azimuth, like Albini's " Gli Azimut 

 del Sole," the angular distance of the sun's meridian, from the south point, for the latitude 

 of the place and the time of observation, may be obtained; and from this, the direc- 

 tion of strike is readilv derived. 



