126 GEOLOGY. 



rock. This pudding-stone, as it is called, we can make 

 such inferences about just as clearly as we can infer the 

 mode of making a plum-pudding from its appearance. 



The geologist goes much farther than this. He dis- 

 covers often in the layers of rock tracks, and even the 

 marks of rain-drops and ripples, made perhaps ages upon 

 ages ago. I will give a single example. Professor Dana 

 says of some slabs examined in Pottsville, Pennsylvania, 

 "We thus learn that there existed in the region about 

 Pottsville at that time (a period just before the coal of 

 that region was deposited) a mud flat on the border of a 

 body of water ; that the flat had been swept by wavelets, 

 leaving ripple marks ; that the ripples were still fresh 

 when a large amphibian walked across the place ; that a 

 brief shower of rain followed, dotting with its drops the 

 half-dried mud ; that the waters again flowed over the 

 flat, making new deposits of detritus,* and so buried the 

 records." 



211. Classes of Rocks. First, rocks are divided into 

 two grand classes, the stratified and the unstratified. 

 The stratified appear in strata or layers, and the surfaces 

 of these strata are nearly or quite parallel. They are 

 either earthy aggregates, as sandstones, or simple chemi- 

 cal precipitates from solutions, as the limestones some- 

 times are. Those which are mere aggregates were de- 

 posited as sediment, and therefore are called sedimentary 

 rocks. They are also called aqueous rocks, as are also 

 those which were precipitated, because water was the 

 agent by which the matter composing them was brought 

 to the locality and deposited. You have an example of 

 stratified rock in Fig. 48. Stratified rocks very general- 

 ly contain fossils that is, remains of plants and animals 

 which were in existence at the time that the material of 

 which they are composed was deposited, and are there- 



* This term is applied to what has been removed from the surfaces 

 of rocks by the erosion of water and other causes. When the material 

 thus removed is coarse, it is called dtbris. 



