8 PALJEONTOLOGY OF KENTUCKY. 



so-called neptimian theory, according to which all the rocks, with the only 

 exception of the volcanic lava formations, were considered as produced by 

 water, or to be of aqueous origin. If this theory had proved correct, the views 

 of the old school about the comparative age of the different rocks would be 

 sustained. But progress in geological science has upset the neptunian theory, 

 and established in its place the plutonic theory, which makes fire or subter- 

 ranean heat the main agency in the production of the plutonic and met- 

 amorphic rocks. 



This new theory does not admit the classification of primary and secondary 

 rocks. The first rocks ever produced, which formed the first thin crust of our 

 globe, were dissolved by water and removed to other localities, where they 

 furnished the constituents of the sedimentary formations. Even a large 

 portion of these have been ground up by weather and water, to provide the 

 material for later deposits. Rocks of all the different classes have been formed, 

 during the past, simultaneously, and may be in process of formation at 

 the present time. The terms primary and secondary are, therefore, obsolete, 

 inasmuch as they indicate the comparative age of the different rocks. 



SEDIMENTARY OR STRATIFIED ROCKS. 



Though all the different classes of rocks are of great interest to the geologist, 

 still the most important of all is that including the sedimentary formations. 

 The rocks of this class are always arranged in layers or strata, and they are, 

 therefore, generally referred to as stratified rocks. They are of greater impor- 

 tance to the geologist than the other formations, because they furnish him the 

 main material for his investigations. Most, if not all, of them have been 

 formed since the beginning of organic life on our planet, as proved by the 

 remains or traces of animal and vegetable organism preserved in their strata. 



The most important among the different points which the geologist has to 

 consider in regard to the sedimentary rocks, are : their mineral composition, 

 their arrangement in strata, their relative age, and, most of all, their organic 

 remains. 



In regard to their mineral composition, we may divide them into three 

 groups, the siliceous, or arenaceous rocks ; the clayey, or argillaceous rocks ; 

 and the calcareous rocks, or limestone. The main constituent of the first 

 group is silica, in the form of quartz-grains or sand ; that of the second group 

 is clay, a mixture of siliceous matter, with a large amount of alumina and 

 oxide of iron, and that of the third group is carbonate of lime. It is impos- 

 sible to separate these three groups by a well defined division line. Some 

 rocks form a kind of connecting link between the first and second, and others 

 between the second and third group, while again, others may combine the 

 siliceous and calcareous rocks. The first group is represented by sandstone, 



