Distinction of Deposits. 39 In one and the fame mechanical’ formation, we obferve a gradation in thefe parts in regard to their magnitude. Thus they occAr as pebbles, gravel, and fand, and the fineft as loam and clay. We fometimes, however, obferve fubttances that are partly of a mechanical, partly of a chemical for mation. Thus there is a kind of Tandftonewhofe bafis or conneding matter is quartz; fo that we have in this inftance a mechanical and chemical formation in the fame rock. The deftroying and forming effeds of volcanic fire on the furface of the earth, will be confidered when we come to treat of Volcanoes^ CHAP. Ill- INTERNAL STRUCTURE OF THE EARTH. ,2. tJAving in the preceding Chapters defcri- JLi bed the various inequalities obfervabl e on the earth’s furface, and ftated the means which nature appears to have employed in forming t em, we come now to the conlideration of the leCond branch of Geognofy, which makes us acquainted with the Internal Strutture of the earth. At firft fight the folid mafs of the earth appears to be a confufed afiemblage of rocky malfes pi-