3o6 



PRIXCIPLES OF STRATIGRAPHY 



Laccoliths differ from sheets in being lens-like in mass, thick in 

 the center and dying away laterally. In the process of intrusion 



W. by N. 



E. by S. 



45 



Section of a composite sill, Island of Skye (after Marker) . The 

 stratified Lias was cut by the sill of basalt ; a later sill, of grano- 

 phyre, was intruded along the middle plane of the basic sill — the 

 latter itself may have been double. 



the overlying strata are arched. Laccoliths may be simple or com- 

 pound, when divided by strong beds of the invaded formation. 

 (Judith Mountains.) They may be multiple or composite (Fig. 46), 



Fig. 46. Section of a composite laccolith. (After Weed and Pirsson.) The 

 laccolith cuts heavy-bedded lava flows. Its maximum thickness is 

 150 feet. Black: basalt; white: granophyre. 



as in dikes and sills. Like sills, they may also be interformational. 

 (Fig. 47.) In form they may be symmetric or asymmetric. These 

 variations are illustrated in Fig. 51. 



Fig. 47. Section of an interformational laccolith. (After Weed and Pirsson.) 

 The floor of the porphyry laccolith (in black) is composed of pre- 

 Cambric crystalline schists ; the cover, of Palaeozoic sediments. 

 The length of the section represented is about ten miles. 



