Handbook of Paleontology 43 



ened. The porosity of a rock depends largely upon the 

 size of the composing fragments: 



When fossils occur in stratified rocks, if the animals 

 were free-living and unattached, as in the majority of 

 cases, they are found lying parallel with the bedding. 

 Attached animals are preserved as attached, in some 

 cases to older rock, which is readily distinguished ; and 

 plants, such as the trees of the Coal Measures, may often 

 be buried and preserved in situ, that is, in the position in 

 which they grew. Pebbles in stratified rocks, if they are 

 few and scattered in a large amount of finer material 

 such as sand, lie flat in the beds whether of beach or river 

 origin. When pebbles are present in large quantities in 



Figure 3 Delta structure: AB topsets ; BC f oresets ; DE 

 bottomsets. 



deposits they assume a position which gives the least 

 resistance to the force of waves or current. On beaches 

 they are often found standing vertically, and in river 

 deposits they are seen imbricating or overlapping one 

 another down stream, and are so found in rocks derived 

 from such deposits. Strata which have not been folded 

 or deformed in any way show the effect of compression 

 from the weight of the overlying deposits when there has 

 been long-continued sedimentation in a region. The sedi- 

 mentary material becomes more compact. Inclusions 

 such as pebbles and fossils give proof of such compres- 



