STUDIES FOR STUDENTS 161 



They may be local in derivation, produced by the breaking up 

 in situ of a terrane, or they may have suffered transportation from 

 distant sources. They may be simple or of complex and brecciated 

 structure, the result of an earlier brecciation. 



Breccia may form a mass entirely destitute of planes of bedding. 

 When bedded in a distinct stratum it may be classified as endo- 

 stratic. 



A crackle breccia, representing incipient brecciation, is one 

 whose fragments are parted by planes of fission and have suffered 

 little or no relative displacement. The fragments match along 

 their apposed sides. The matrix is confined to the seams and is 

 commonly a chemical deposit. 



A mosaic breccia is one whose fragments have been largely but 

 not wholly disjointed and displaced. The system of continuous 

 cracks of the crackle breccia has been destroyed, but more or less 

 of the fragments still match along adjacent surfaces and show that 

 they are consanguineous parts of once unbroken laminae or larger 

 beds. The term suggested is not a happy one, yet these breccias 

 may recall some ill-preserved mosaics of ancient ruins. The 

 matrix is confined to the seams and to the wider and irregularly 

 shaped interstices. 



A rubble breccia is one in which no matching fragments are 

 parted by initial planes of rupture. The fragments are close-set 

 and in touch. 



A breccia of sporadic fragments is one in which the fragments 

 are imbedded in a preponderant matrix like plums in a pudding. 

 It recalls the term "plum-cake rock," applied by the quarrymen 

 of North England to the breccias of their region. In his study of 

 the Permian Midland breccias of England, King 1 distinguishes 

 an endostratic breccia of this class by terming it "breccia sand- 

 stone," thus emphasizing the bedded matrix rock. 



Breccia may be fossiliferous. Fossils may be restricted to the 

 fragments or to the matrix or may be found in both. Fossils of 

 the matrix may themselves be fragments. Brecciation may be 

 practically contemporaneous with the involved deposits. Such 

 are the intraformational breccias of Walcott. In certain classes 



I W. W. King, "Permian Conglomerates of the Lower Severn Basin," Quar. 

 Jour. Geol. Soc. London, LV (1899), 105. 



