4S0 Prof. Sedgwick on the Structure of large Mineral Masses. 



nomenclature, but a more systematic use of old terms than we are accustomed 

 to. Bed is always applied as the English synonym oi stratum, and the terms 

 thick-bedded, thin-bedded, thick-flag g7/, thin-flaggi/, and laminated, are words 

 in common use, and express, well enough^ different modifications of stratified 

 structure. The ievm foliated, again, expresses very well the peculiar struc- 

 ture of mica schist, and the fine glossy undulating layers of greywacke. But 

 it would be well to describe no structure as slatj/ or fissile except cases of 

 transverse cleavage, using the term slate for a perfect oblique cleavage, and 

 some such term as flagstone-slate for imperfect cleavage ; and_, in like manner, 

 slaty flagstone may describe a very thin or laminated structure, parallel to the 

 stratification. In this way, foliated as distinct from laminated, and slaty as 

 distinct from flaggy, become terms of a definite meaning-. 



When a branch of natural history has been placed on a settled basis, it is 

 well to use such classical terms and definitions as may be current throughout 

 the scientific world. But while a science is in progress, and its principles un- 

 fixed, the aftectation of regular definitions and technical terms of classical ety- 

 mology may do more harm than good, especially if they be derived from an 

 hypothesis. Were such words as pyroxene and protogine mere jargon without 

 meaning, we might then retain them as proper names, which, however bar- 

 barous, cannot be conveniently parted with. But they have a meaning which 

 proves them to be the offspring of ignorance and error, and on that account 

 they ought to be expelled without mercy from our pages. The pedantry and 

 the gross absurdity of many terms in mineralogy are obvious to every one ; 

 and I hope we shall take a lesson from the progress of that kindred science, 

 and not be too hasty in seeking the useless decorations of a too classical 

 nomenclature. Indeed, there already appear in our descriptions some words 

 of doubtful and portentous etymology, with Grecian heads and Gallican tails, 

 which figure but oddly amidst the ordinary staple of our homely pages. 



§ 4. Jointed Structure. 



Many rocks, both stratified and unstratified, are divided into solids of 

 greater or less regularity, by parallel systems of fissures or joints. This gives 

 rise to a jointed structure, and is quite distinct from slaty cleavage. For the 

 joints are at definite distances from each other, and a mass of the rock be- 

 tv/een them has, generally speaking, no tendency to cleave in a direction 

 parallel to them. The structure in question seems in most cases to have been 

 produced mechanically, either by a strain upon the rock from external force, 

 producing, more or less, regular sets of cracks and fissures, or by a mecha- 

 nical tension on the mass (produced probably by contraction) during its pass- 



