CONGLOMERATE. 135 



all the limestones. It is easy to ascertain whether a rock 

 be calcareous or not, by applying dilute nitric or sulphuric 

 acid : if it effervesces, it is calcareous ; but if not, it is 

 siliceous. A mixture of sand and clay is usually termed 

 loam, and the combination of calcareous matter and clay 

 is called marl ; but both these expressions are employed in 

 a vague sense, and applied to substances of very dissimilar 

 kinds. 



STRUCTURE or SLATY EOCKS. There are three distinct 

 forms of structure frequently present in rocks of this nature : 

 stratification, joints, and slaty cleavage ; the last two having 

 no connexion with true stratification. Joints are natural 

 fissures of various sizes, from mere cracks to open chinks, 

 which often traverse rocks in straight and well-defined lines ; 

 while slaty cleavage consists of lines or partings, which form 

 the point at which the stone may be readily divided. Joints, 

 are distinguishable from slaty cleavage, by the fact, that 

 the rock intervening between two joints has no tendency 

 to cleave in a direction parallel to the planes of the joints ; 

 whereas, a rock is capable of indefinite subdivision in the 

 direction of its slaty cleavage. As the origin of joints and 

 slaty cleavage constitute problems of considerable difficulty, 

 on which geologists are by no means agreed, it may be 

 sufficient to state, that as these characters are only observable 

 in rocks of crystalline texture, and are most strongly defined 

 in such as are ascertained to have been acted on by heat, 

 we are justified in inferring, that heat has been the chief 

 cause of these results, under whatever conditions it may 

 have operated ; and, as a general view, it may be considered 

 that slate rocks having been originally deposited in hori- 

 zontal planes, the heat has been such as to obliterate 

 these planes, and substitute minute vertical divisions in 

 their stead. 



BRECCIA. This name is commonly applied to those sub- 

 stances which consist of sharp, angular, unworn pebbles 

 cemented into a mass. 



CONGLOMERATE. This term is usually bestowed on con- 

 cretions of a similar kind, but in which the imbedded frag- 

 ments are rounded and waterworn. 



GROUP. This name is generally applied to a series of 

 rocks of the same mineral composition and geological 



