164 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
LITERATURE. 
The published knowledge relating to joints is readily divisible 
into two groups, that concerning joints in stratified rocks, and that 
concerning like fractures in igneous rocks. From these two divi¬ 
sions depending upon the difference in the physical history of the 
two great groups of rocks, have grown up two classes of experi¬ 
menters and students of joints. The literature concerning joints in 
stratified rocks deals with the direction and inclination of these 
planes, their extent and stratigrapliical relations, their bearing upon 
orogeny, and their probable cause and their artificial production by 
the application of force. The hypothesis of De Beaumont and later 
the writings of Hopkins gave, in the second quarter of this century, 
an impetus to the study of joints which was limited, however, mainly 
to collecting data as to their strike and dip and their relation to 
mountains. Phillips, Jukes, De la Beche, O. Fisher, and later 
Daubibe and Crosby have contributed to the various phases of the 
subject, the last two more particularly of the artificial production of 
joints by torsion and shock. In none of the authors mentioned or 
in the more recent publications of Becker does there appear a 
description of the intimate structure which is commonly found to 
pervade the rock adjacent to the individual joint and to have a 
topographic expression on the joint-plane itself. 
Accepted Opinions regarding Joints . 
The general facts regarding the occurrence of joints in stratified 
rocks have long been widely known. Joints occur in sets either 
parallel or slightly divergent; usually there are two or more sets, 
two of which tend to have a rectangular intersection. From these 
varying intersections arise quadrangular, triangular, and polygonal 
joint-blocks. 
A joint may traverse all the strata visible in an exposure or quarry, 
being indifferent to alternations of material or texture. Other joints 
are limited to a single stratum, thick or thin. 
Joint-planes are, so far as the literature is concerned, apparently 
taken to be structureless, smooth surfaces, and in text-books are 
commonly represented as unlimited in any direction. It is needless 
to cite references. 
The special features to which attention appears not to have been 
paid are two, viz., the minute fracture-planes by the combination of 
