No. 9.— On the Fracture /System of Joints, with Remarks on 
tain Great Fractures. 
Cer- 
By J. B. Woodworth. 
i 
INTRODUCTION. 
The “ Cambridge slates ” exposed in the quarries facing the Mystic 
River in Somerville, Mass., afford an opportunity for the study of a 
variety of secondary structures commonly known as joints. The 
strata are relatively tliin-bedded or banded pelites, with alternations 
from thick, fine-grained sediments to layers of gray and often 
coarser bands. The lower portion of the section is more argilla¬ 
ceous and compact, the upper more arenaceous and less dense. 
True slaty cleavage is very uniformly wanting in the exposures 
along the river, except for its appearance as a minor structure 
pervading certain layers. These strata together with the outcrops 
lying westward to the Charles River are commonly known as “slate,” 
but in the sense that slate is a rock with slaty cleavage, they are in 
a strict sense not slates. The dominant divisional planes are, in the 
lower group of beds, closely set joints. In the upper exposures, the 
secondary divisional planes are mainly coarse, shaly fractures, sub¬ 
parallel, but discontinuous and running into each other. In this 
paper, I shall call the rock a petite, 1 a term which implies no struc¬ 
ture other than that of original stratification. The rocks are the 
argillite of Dr. Wadsworth, but this term is now also apt to mislead 
since the French use of the word.applies to roofing slates. There 
is wanting a term correlated with shale, slate, and schist, which 
shall designate a rock breaking up into fragments of variable but 
frequently of small size bounded by joints. Among the quarrymen 
of the much-jointed Pre-Cambrian limestones of Lincoln, R. I., I 
have been informed by Professor Packard, the name “ jointer ” is in 
use. If one of the numerous terms given to the indurated clay rocks 
and used as equivalent to slate were to be retained for finely jointed 
rocks without slaty cleavage, a decided gain would be made in the 
terminology. 
1 A. Geikie : Text-book of geology, 1893, eel. 3, p. 132. 
