450 SAODIES TORS S AOD ENTS, 
that ordinarily given in the text-books, but it is believed that 
the restriction is a gain for accurate discussion. The structure 
” 
corresponds to Sorby’s “ultimate structure” cleavage." 
Fissility is here defined as a structure in some rocks by vir- 
tue of which they are already separated into parallel lamine in 
a state of nature. The term fissility thus complements cleav- 
age, and the two are included under cleavage as ordinarily 
defined. Where a rock is finely fissile it may be called foliated. 
Fissility corresponds in part to Sorby’s ‘‘close joints” cleavage.3 
The terms slate and schist, slaty and schistose, slatiness and 
schistosity, are retained with their usual significations. A slate 
may be defined as a rock having the property of cleavage or 
fissility, or both combined, the rock parting into layers with 
relatively smooth surfaces. In slates the mineral particles are 
usually of small size. A schist is a rock having the property 
of cleavage or fissility, or both combined, the rock parting into 
layers with rough or wavy surfaces. In schists the mineral par- 
ticles are larger than in slates. As is well known, there are all 
gradations between slates and schists. In origin and essential 
characters the two have many properties in common.  Schis- 
tosity, however, indicates more severe metamorphism than slati- 
ness. It follows, from the above definitions, that a slate or a 
schist may have the property of cleavage or of fissility or of 
both combined. When both are present they may be parallel 
or intersecting. Both may occur in the same slate or schist in 
more than one direction. 
It is not the aim here to inquire fully Vas) to) thejman- 
ner of development of cleavage and fissility. Starting with 
Une mesuitts iteacliecl joy JPlniulhios,? Sirejoe,® Sorroy,° Wyin- 
*On Some Facts Connected with Slaty Cleavage, by H. C. Sorsy, Rept. British 
Association for the Adyancem’t of Sci., 27th meeting, 1857, pp. 92-93 of Transactions. 
? Report on Cleavage and Foliation in Rocks, and on Theoretical Explanations of 
these Phenomena, JOHN PHILLIPS, British Association for the Advancement of Science, 
26th meeting, 1856, Proceedings, pp. 369-396. 
3On Slaty Cleavage, DANIEL SHARPE, Quar. Jour. Geol. Soc. Lond., 1846, Vol. III, 
pp: 74-105; Vol. V, 1849, pp. 111-129. 
4Anniversary Address of the President, HENRY CLIFTON SORBy, Quar. Jour. 
Geol. Soc. Lond., 1849, Vol. XXXVI, Proceedings, pp. 68-92. 
