I. Introduction. The object of this paper is to give a detailed description of a rock consisting essentially of quartz, which occurs in the Mountain Limestone of Derbyshire. It is not a quartzite in the ordinary meaning of the word—that is, a sandstone indurated by the deposit of interstitial quartz or by the recrystallization of the detrital grains into a clear mosaic, though sometimes a specimen may be found simulating the latter structure. It belongs rather to the second division of quartzite suggested by Mr. Rutley, namely, that of an infiltration or metasomatic quartzite; in other words, it is a silicified limestone. The term ‘quartz-rock’ will, therefore, be applied to it in this paper. Often associated with the quartz-rock is a quartzose limestone. This is a fossiliferous limestone containing a large number of quartz-crystals, which may be easily seen with a lens when the rock is wet. I first noticed the quartz-rock when commencing my study of the Toadstone some years ago. Near the village of Bonsall, the footpath from Ember Lane to Pounder Lane passes a bold outcrop of hard rock, locally known as the ‘Top Lift.’ A thin section from it was prepared, and on examination was found to consist of quartzgrains. Until about 2 years ago it lay in my cabinet. Since 1894 I have been engaged during my spare time in re-mapping the Toadstone and in trying to differentiate successive horizons in the limestone in order to decipher, if possible, the history of the volcanic eruptions