100 OVERLYING AND TRAP ROCKS. 



not been observed, or not described. But no other 

 essential innovation having been made, the different 

 members are as follows ; Claystone, Clinkstone, Com- 

 pact felspar, Basalt, Greenstone, Syenite, Augit rock, 

 Hypersthene rock, Porphyry, Amygdaloid, and Tuff. 

 The term trap has been applied to all these sub- 

 stances indiscriminately, from ignorance of their mi- 

 neral characters, or to save the trouble of investiga- 

 tion ; while, as used by foreign geologists in particular, 

 its meaning is often so vague as to be unintelligible. I 

 have no scruple in rejecting a word which, while it 

 is the cloak for ignorance, perpetuates it ; but being 

 a convenient generic one in topographical and general 

 description, I have adopted it for the whole family in 

 the secondary class. The essential minerals in all 

 these rocks are, indurated clay, clinkstone, compact 

 felspar, hornblende, augit, hypersthene, and common 

 felspar. The unessential and casual ones will be 

 found in the Classification of rocks. 



The predominant substance in the members of this 

 family, is a simple rock ; commencing with indurated 

 clay, or CLAYSTONE and terminating with COMPACT 

 FELSPAR ; the intermediate members being INDURATED 

 CLAYSTONE and CLINKSTONE. These sometimes form 

 the whole mass ; while, at others, they are mixed with 

 other minerals, so as to produce many of the com- 

 pounds in this multifarious tribe. Chemistry has not 

 proved a real transition between clay and compact 

 felspar ; so that the present arrangement is founded 

 on the apparent transitions in nature ; all these sub- 

 stances occurring together under the same geological 

 unity, often in the same mass, and passing into each 

 other without definable limits. The term compact 

 felspar, is not judicious ; from well known circum- 

 stances in the composition of this mineral and coin- 



