substance, and obviously mechanical structure, as the essential part of the definition. I shall take an opportunity in some remarks on another district, to enquire whether it would not be also convenient to extend the definition so far as to permit mica slate to participate with clay slate in the office of cement, the other parts of the character remaining the same.
Whatever definition be ultimately adopted, we cannot too strongly inculcate the necessity of accuracy in the application of terms. Accurate mineralogical knowledge is an indispensable condition to accurate geological description, and the errors of very modern and celebrated authors, arising from the want of this fundamental quality, are too well known to call for the invidious task of pointing them out.
The cacophony of the term graywacke has excited a desire in some late observers to discard it altogether, and substitute one more vernacular, and less liable to that objection.
The multiplication of synonyms is itself an evil of so crying a nature, and has unfortunately become a disease of such magnitude in our science, that we ought to consider well before we venture to add another to the unwieldy and vexatious stock. It is so great an advantage to possess one term well understood, and understood as the language of philosophy should be, in all countries, that we can scarcely find a motive sufficiently powerful to induce us to change this received name. Under these circumstances we have every reason to retain the term graywacke, however jarring to English ears, and it is the excess of fastidiousness to reject on account of its sound, one word from such a polyglot of unmelodious and ill compounded Greek, French, and German terms, as assail the mineralogist on every side. If there is in any case a choice among equally established terms, it is in our power to chuse