At the Byne Hill, near Girvan, a large intrusive boss or ridge displays on its outer margin a fine-grained texture, where it comes in contact with the serpentine. Further inwards it becomes a fine dolerite, passing into gabbro and increasing in coarseness of grain as well as in acidity of composition, through stages of what in the field would be called diorite and quartz-diorite, into a central granitic rock, whereof milky or blue quartz forms the prominent constituent. The intrusive rocks of this district have generally been injected parallel to the stratification-planes, and take on the whole the form of sills. Some time after the close of the volcanic episode in the Silurian period of the south of Scotland, the rocks were locally subjected to considerable disturbance and elevation, whereby parts of the volcanic series were exposed to extensive denudation. Hence the overlying unconformable Caradoc conglomerates are in some places largely made up of the detritus of the volcanic rocks. It is interesting to find this evidence of waste during the very next stage of the Silurian period, for it affords good evidence that the extensive sheets of intrusive material could not have had any large amount of overlying strata resting upon them at the time of their injection. Pieces of these intrusive rocks, such as the serpentine, occur abundantly in the Caradoc conglomerates, some of which indeed are almost wholly composed of their detritus. Probably the total thickness of the overlying cover of rock under which the sills were injected did not amount to as much as 200 or 300 feet. Yet we see that among the sills were coarse gabbros and granitoid rocks. We may therefore infer that for the injection of such intrusive masses, great depth and enormous superincumbent pressure are possibly not always necessary. During the progress of the Geological Survey along the southern borders of the Highlands, a remarkable group of rocks has been observed, intervening as a narrow interrupted strip between the schistose masses to the north and the great boundary-fault which brings the Old Red Sandstone in vertical strata against them. Between Cortachy in Forfarshire and Stonehaven on the east coast, these rocks have been mapped by Mr. G. Barrow, who has carefully worked out their relations. They appear again between Callander and Loch Lomond, where their extent and structure have been mapped by Mr. C. T. Clough. For the purpose of our present inquiry two chief features of interest are presented by these rocks. They include a group of sedimentary strata among which occur bands of jasper or chert containing radiolaria, and one of their most conspicuous members is a series of volcanic rocks consisting chiefly of dolerites and basalts, some of which have been much crushed and cleaved, but in which vesicular structures can still occasionally be recognized.