This widespreading change may extend over great regions and vast extents of country. The most striking series of such rocks was first described by Sir W. E. Logan, Director of the Canadian Geological Survey; and he estimated the thickness of them at 30,000 feet. They lie beneath all the unaltered rocks, and are (in North America) the rocks which were the base or foundation of the North American continent before the later sedimentary rocks were laid down on them. They are called the Laurentian rocks, because they were first found in the neighbourhood of the St. Lawrence River; but they exist in many places besides Canada and North America; and the foundations of Scandinavia and of the Hebrides are of the same texture and material. Now this change or metamorphism does not appear to be the same as that produced by the intrusion of hot eruptive rocks. Let us take a simple instance. We have seen that limestone is changed by heat into marble. Sometimes its fossils are preserved; sometimes they completely disappear. Sometimes it is threaded by veins of harder and more crystalline rocks. But in the case of the white marble of Carrara, which was once a bed of coral, the change seems to have taken place less violently, less suddenly, more gradually. The change was due, therefore, not to violent heat suddenly applied, but to the penetrating action of water, probably aided by sustained heat, and certainly aided by pressure. When a rock is subjected to sufficient pressure its very structure will alter; its original constituents may be torn out of it, pressed out of it, filtered out of it, and afterwards rearranged.