CONDITIONS AFFECTING ROCK DECAY 325 



ing is perhaps the most important. It pervades nearl}^ all the rocks of 

 the region, both igneous and sedimentary. The joints which intersect 

 the igneous rocks are perhaps largely due to shrinkage on cooling. The 

 regular prismatic jointing common in basaltic lava flows has not been 

 observed in this region. In its stead is a system of more or less regular 

 joints which divides the rock into large rhomboidal blocks. The less 

 basic rocks, such as the dacite and the volcanic conglomerates, are nearly 

 or quite free from these joints, and the manner in which they weather 

 is therefore quite different from that of the basalt. 



The sedimentary formations are generally very deeply fractured. In 

 these the joints are doubtless due to the action of dynamic forces, which, 

 while they have not greatly changed the original position of the beds? 

 have been sufficient to thoroughly shatter them to great depths. Only 

 a few of the more massive beds of sandstone have in some measure 

 escaped this general fracturing. Its effect i^ most pronounced in the 

 less massive portions of the Brito formation. At the surface the joints 

 have been enlarged by the weathering process, and the rock consists of 

 a loose mass of small fragments. This condition prevails to a depth of 

 more than 100 feet from the surface, as shown by the boring at La Flor. 

 In some cases it was observed that the cracks which intersected the 

 rocks had subsequently been healed up by the deposition of calcite. 

 This, however, is not general at ordinary depths. 



A direct consequence of the presence of these cracks intersecting tlie 

 rocks is the development of secondar}'^ concentric structures. The cracks 

 permit the percolation to great depths of surface waters bearing the 

 agents which are most active in rock decay. The weathering proceeds 

 outward from these joints with the production of successive concentric 

 la3^ers about a central nucleus. The concentric structures which have 

 already been described were thus produced. 



ROCK DECAY IN THE EASTERN DIVISION 



As has been already pointed out, there is a marked difference in the 

 distribution of the rainfall on opposite sides of the isthmus, with a cor- 

 responding difference in the character of the vegetation and in the extent 

 and products of rock decay. It will be necessary, therefore, to consider 

 the process and the products in the two divisions of the isthmus sep- 

 arately. 



The eastern division is characterized by a heavy rainfall, so distributed 

 throughout the year that there is no well marked dry season ; hence the 

 surface soil is never permitted to become dry and the forest litter is 

 never removed by fires. The entire surface is covered with a dense 



XLVII— Bull. Geol. Soc. Am., Vol. 10, 1898 



