They were supposed to have been formed while animals and vegetables existed in numbers, and to have been partly chemical and partly mechanical in their origin. The fourth class contained the alluvial rocks, those produced on the land, as peat, sand and gravel, loam, bog iron ore, calc tuff, etc, being understood to comprise all above the chalk excepting the volcanic. The fifth class comprised the volcanic rocks, the pseudo-volcanic, and the true volcanic; the former being the supposed products of the combustion of coal and sulphurous matters, the latter of real volcanoes. These formations were supposed to be systematically arranged; the later formed either entirely covering the older, or, when these form a central mountain mass, encircling this, so that the "outgoings" of the strata (meaning their upper edges or lines of outcrop) form circles; those of the later formed groups being successively larger. The basin and trough-shaped deposits were also recognized, in which the outgoings of the newer strata became successively smaller. The strata, it was understood, were subject to local disturbances from portions sinking into subterranean cavities, and members might be wanting in some localities, but whenever present must be found in their proper position in relation to the others.

Basalt, which in Saxony and Hesse was seen capping the hills of stratified rocks, he inferred must be of the same series of precipitated formations, although many other geologists of Werner's time had fully established the analogy between this rock and modern lavas. The observations of Desmarest, especially in the district of extinct volcanoes in Auvergne, made in 1768, are referred to by Lyell as most clearly tracing the origin of the basalts to the craters of the volcanoes. A new controversy now arose, which for many years was waged with animosity and bitterness unprecedented in disputes of this class. Geologists throughout Europe were divided into the two classes of Neptunists, who advocated the production of the rocks by aqueous deposition alone, and Vulcanists, who attributed the origin of many of them to the action of fire. They were also called, from the names of their respective leaders, Wernerians and Huttoni-ans. Dr. Huttcn of Edinburgh had studied geology for himself in different parts of Scotland and England, and formed his own conclusions, which he ably sustained.

He was the first to announce that geology had no concern with questions as to the origin of things, but that the true field of its investigations was limited to the observation of phenomena and the application of natural agencies to explain former changes. His friend Sir James Hall showed by actual experiment that the prismatic structure of basalt might result in cooling from a state of igneous fusion; and Hutton himself found in the Grampian hills the granite branching out in veins, which extended from the main body through the contiguous micaceous slates and limestone, thus indicating its having been in a fused state at a time subsequent to the production of Werner's primitive rocks. This discovery soon led to questioning the existence of any primitive class of rocks the origin of which lay beyond the reach of the present order of things; and the announcement made by Hutton, In the economy of the world I can find no traces of a beginning, no prospect of an end," may well have startled men of science and shocked the religious public in the sensitive condition to which it had been brought by the infidel doctrines promulgated in the latter part of the last century, especially by men of letters in France. The Vulcanists came to be classed with the enemies of Scripture, the true object of investigation was lost sight of, and the controversy was continued with such animosity that the party names at last became terms of reproach, and many geologists avoided being involved in it.

Workers in the field, however, were collecting new and valuable data that were to give to the science a more exact character. William Smith, a civil engineer, prepared in 1793 a tabular view of the strata near Bath, tracing out their continuity over extensive areas, and recognizing them by the fossils they contained. This method of identification and of arranging strata in their true positions he taught himself, and was the first to promulgate in England. With extraordinary perseverance he continued to prosecute his work alone, travelling on foot over all England, freely communicating his observations, and in 1815 he completed a geological map of the whole country. In France the importance of fossils as characteristic of formations was also beginning to be appreciated. Lamarck and Defrance earnestly engaged in the study of fossil shells, and the former in 1802 reconstructed the system of conchology to introduce into it the new species collected by the latter in the strata underlying the city of Paris. Six years previous to this Cuvier had established the different specific character of fossil and living elephants, which opened to him, as he said, views entirely new respecting the theory of the earth, and determined him to devote himself to the researches which occupied the remainder of his life.

In 1807 the geological society of London was established, with the professed object of encouraging the collection of data, multiplying and recording observations, with no reference to any "theories of the earth." Its active members completed the classification and description of the secondary formations of Great Britain, so well commenced by William Smith; while at the same time the tertiary formations were thoroughly investigated by Cuvier, Brongniart, and others in Paris. Thus each country contributed to the advancement of geological science in the department connected with its most prominent formations: Germany in that of the lower stratified and crystalline rocks, and especially in the mineralogical structure of these, while in Scotland the character of the granitic rocks had been more particularly elucidated, in England that of the secondary strata and their order of arrangement, and in France the tertiary. The great principles gradually developed by these observations were: that the materials of the stratified rocks were sedimentary deposits that had slow-ly accumulated in the beds of ancient seas and lakes; that each stratum represented a certain period during which its materials were gathered, and that this period was characterized by its peculiar group of organized beings, the vestiges of which were buried and remained with it as records of the condition of this portion of the earth during this time.